Added a helper method to check if required parameters are given in an API call. Can be used
to return a `400 Bad Request` return code if a required attribute is missing.
Code clean up and fixed tests.

The API documentation of merge requests contains info to status codes for all functions.
Required arguments are now checked in the merge requests API functions and a `400 Bad Request` error is
returned if they are not given.

closes #2300

The issues documentation is updated with infos to status codes and the deprecated `DELETE` function and
how to close an issue. A few more tests added to check status codes of API functions.

Updates the API documentation of groups with infos to return codes. The function calls
in the groups API have updated documentation and return `400 Bad Request` status code
if a required attribute is missing.

The API documentation of repository is updated and now contains infos to status codes.
Code documentation is also adjusted for `GET /projects/:id/repository/commits` and includes infos to
pagination attributes. Tests are updated.

Updated the milestones API documentation and added return codes descriptions.

The notes API documentation updated with return codes. API now returns `400 Bad Request` if
required attributes are not present. Return codes are documented now, also tested in added tests.
The documentation now reflects the current state of the API.

The users API updated with return codes, e.g. if required parameters are missing
a `400 Bad Request` error is returned instead of `404`. Fixes return codes of functions,
e.g. deletion of a ssh key is an idempotent function now.
The API documentation is updated to reflect the current status of the API. Descriptions
are more detailed and complete, infos to return values are added to all functions.

When a user is not authorized to see the list of hooks for a project, he is
still able to access the hooks separately. For example if access to
`GET /projects/:id/hooks` fails and returns a `403 Unauthorized` error it is
still possible to access a hook directly via `GET /projects/:id/hooks/:hook_id`.

Fixes access, also added tests to check access and status codes of hooks.

Status code 422 (Unprocessable Entity) returned if invalid url is given when creating
or updating a project hook.

When the project limit is reached the user is not allowed to create new ones.
Instead of error code 404 the status code 403 (Forbidden) is returned with error
message via API.

The previous call `saved?` is restored in the `POST /projects` method in the API.
It is refactored to check if the record is persisted. This is useful to not validate
the record again after saving. This fixes the returned status code in the web client
too. If the last project is created via web client instead of error notification
the project page is shown.

When creating the last project via API when reaching the project limit a status code
of 404 (Not found) is returned instead of 201 (Created). The fix checks now correctly if
the project could be saved.

Extracted a method for 400 error (Bad request) and adjusted code accordingly. The name of
the missing attribute is used to show which one was missing from the request. It is used to
give an appropriate message in the json response.

When using project snippets via API the functions now provide status codes for
different situations other then only returning 404 error. If required parameters are missing,
e.g. `title` when creating a project snippet a 400 (Bad request) error is returned. The snippet
delete function now is idempotent and returns a 200 (Ok) regardless if the snippet with the
given id is available or not. Changing return codes of these functions has the advantage that
the 404 error is used only for resources, which are not available.

Tests added to check these status codes when handling project snippets.

Different status codes in the API lib are returned on hook creation, update or deletion.
If a required parameter is not given (e.g. `url` in `/projects/:id/hooks/:hook_id`) status
code 400 (Bad request) is returned. On hook deletion a 200 status code is returned, regardless if
the hook is present or not. This makes the DELETE function an idempotent operation. Appropriate tests
are added to check these status codes.

Instead of using funtion `error!` the function `not_found!` is used to return 404 error. Adjusted documentation accordingly.

Accessing a repository branch that does not exist returns a 404 error instead
of 200 now. Added a test.

When adding a project hook a url must be specified or a 400 error code is returned

* Specs added to check status code on handling project hooks
* refactored code, extracted a method

When a user is added to a project that is already a member of, a status code 201 is now returned to
signal an idempotent operation. If something fails then instead of returning error code 404 different
more specific error codes are returned. Status code 400 (Bad request) is returned when a required
attribute, e.g. `access_level` is not given or 422 if there is a semantic error, e.g. should
the `access_level` have an unsupported value.

Specs are added to check these status codes.

If a note is created with a POST request via API (`/projects/:id/notes`) status
code 400 is returned instead of 404. The resource itself exists but the request
is incomplete. Specs added to check different status codes when accessing, creating
and updating notes.

If a milestone is created via API but no title given then status code 400 (Bad request)
is returned instead of 404. A small helper method handles the errors collection of a
milestone.

Creating a comment to an existing merge request via API without providing a note
returns a status code 400 now, suggesting a bad request. The reason for this
is the resource itself (MR) exists but the required property is not set.

Using the API library to create or update a merge request at the moment a 404 error is returned.
This is fine when the merge request in question does not exist, but does not provide good
information that for example a required attribute is missing.
A status code of 400 (Bad request) is returned when creating or updating a merge request
when either `source_branch` or `target_branch` is missing. A status code of 409 is returned
when `source_branch` and `target_branch` are the same. Tests are added for these cases.