Added ability to add and remove the forked from/to relatioinship
between existing repos.

s 3599: Add snippets_enabled flag to projects for API

Replace missing comma

Add api to collect owned user projects. Api deploy_key.create: Enable deploy key if it exist in other owned project

Also specify separate project factory for project with filled repo

This allows you to set the namespace ID for projects via the the API. By default it is created for the current user. You can assign it to the global namespace by passing `GLN` which translates to 'Global Namespace'.

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.

closes #2300

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.

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.