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.

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.

Some fix for gitlab:gitlab_shell:check

Fixing issue #2970

Update lib/tasks/sidekiq.rake

API: Fixes return code when accessing non existent branch (#2922)

Merge branch 'easy-to-find-commit-on-network-graph' of https://github.com/hiroponz/gitlabhq into hiroponz-easy-to-find-commit-on-network-graph

Disable autocomplete for admin/users form

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.

Mac OS uses launchd instead of /etc/init.d to start daemons and tasks to be started by launchd MUST NOT daemon itself. So "nohup" here won't work for Mac OS. 

Can we add a "launchd" task to the rake file so that we can start sidekiq as "bundle exec rake sidekiq:launchd" ?

Gitlab incorrectly routes blobs to the compare controller depending on filename.

Merge branch 'fix-db-after-updating-rails' of https://github.com/Undev/gitlabhq into Undev-fix-db-after-updating-rails