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).