Add check if the signup setting is enabled. fixes #4832

Add hints for relative URL installation

Fixed typo in groups.md

Extended User API to expose admin and can_create_group for user creation...

Changed wording in application.rb and gitlab.yml.example to match unicorn.rb. This is for when using a non-root path

* dont allow protect/unprotect branches for users without master permissions
* dont allow access to Repository api for guests

add rake tasks for web hooks management

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).

run mysql_secure_installation to secure the freshly installed mysql server

[gitlab:check] Fix detecting the status of sidekiq

If you are running another sidekiq instance on your server, e.g. GitLab
CI, the check script would parse the output of `ps aux` searching for
`sidekiq` and returning success, although the GitLab sidekiq may not be
running.

Now the `ps` call will only print the processes run by the GitLab user.

Added reference to the line "ENV['RAILS_RELATIVE_URL_ROOT'] = "/gitlab"" , as it talked about in the gitlab.yml.example