Needed to support Huboard

Added ability to update or set the identity of an existing user, like the documentation said it was possible, but actually wasn't.

Closes #2267

Closes #1856
Closes https://github.com/gitlabhq/gitlabhq/issues/9394

To prevent loose of group data you need to transfer or remove group
first before you can remove user
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

* users (#6878, #3526, #4209): Validation error messages are now exposed through 400 responses, 409 response are sent in case of duplicate email or username
* MRs (#5335): 409 responses are sent in case of duplicate merge request (source/target branches), 422 responses are sent when submiting MR fo/from unrelated forks
* issues
* labels
* projects

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

To prevent leaking of users info we reduce amount of user information
retrieved via API for normal users.

What user can get via API:

* if not admin: only id, state, name, username and avatar_url
* if admin: all user information
* about himself: all informaion
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Complements POST operation added in gitlabhq/gitlabhq#3146

Implement DELETE /users/:uid/keys/:id for admin users

Fix "Line is too long. [83/80]"

Use single quotes as advised

Use single quotes as advised

Use single quotes as advised

Fix missing space around { and }

Fix typo in documentation

Only catch  ActiveRecord::RecordNotFound, let other exceptions propagate

Raise a "404 Not found" if key to be deleted cannot be found
As requested by @jvanbaarsen in https://github.com/gitlabhq/gitlabhq/pull/6781#discussion_r11735114

Remove tab
Unconfigured vim on this box, grrrr./

Also, is_admin and can_create_group are exposed in the user information.
Fixed attributes_for_keys to process properly keys with boolean values (since false.present? is false).

-calling build_user will now apply defaults and only override them if as: :admin is set

Change-Id: Id1d938c0967752ecc14370af54f2d88128d18c44

-API now respects default_projects_limit, default_can_create_group, and default_can_create_team

Change-Id: I059d060d576df1050e5371e707381c5e8c608a7a

Most of these are comments but a few are strings for users.

Might be an idea to run this from time to time:
https://github.com/lyda/misspell-check

It runs mostly clean now.

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 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.

Also added tests.

The 2 reasons are :
- creation of user fail if name is empty : in after_save, it tries to create a
namespace with namespace.name = user.name and namespece validates presence
Namespace#name
- in the web app links to team members are broken with empty User#name because
they are of the form <a href'...'> user.name </a>