API: tests to show incorrect behavior when reaching project limit

When reaching the project limit the API returns an error code 404 on the last possible
project. The project itself is created and is available in the database (seems valid).
A similar behavior can be observed when reaching the project limit via web client, but
in this case the user is notified that the maximum number of projects is reached. The project
itself is still created and can be accessed.

Tests are added to check the behavior when reaching the project limit or one tries
to exceed it via the API.
API: fixes return code when creating last project before reaching limit

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.
API: refactored last fix, project limit in web client is fixed too

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.
Change .gitignore to ignore logrotated log files.
Add option to disable username changing

This option allows to disable users from changing their username.

This is very usefull in environments using strong internal authentication methods like ldap, pam or shibboleth.

You can allow users to change theyr username in these environments, but then new users (users loging in first time) is blocked from gitlab is her username exists.
Fix ssh key fingerprinting test to use exitcode
Connect to database with git user, not gitlab
Run db:setup and db:seed_fu before gitlab:setup
Added relative urls to other documentation files, removed duplicate debian update commands
Nginx: edit actual file instead of symlink
Added creation of pids directory to fix #2995, replaced deprecated nginx init.d command with service one, some more clean-up
API: fixes visibility of project hook

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.
Sort groups alphabetically on dashboard page
Sort groups/namespaces by human name
Fix link to owner of group and team in admin interface
Merge pull request #3017 from jojosch/owner-link-admin

Fix link to owner of group and team in admin interface
use attachment secure_url for Attachemnt page
Tests to validate that invalid keys are rejected
Fix ssh key test by adding valid ssh key
Prevent xss attack over group name. Added regex validation for group and team name