PLA-1823 Disable Promote for non-staff

* Introduced new permission group ( promotedisabled )
* Rewired Promote to check for permissions to promotedisabled
* Introduced i18n message containing the informational text
* Removed AdminDashboard CSS asset
PLA-1823 Copy change

* Changed copy
* Removed outdated comment
PLA-1823 Remove Promote from AdminDashboard
PLA-1823 Remove username from i18n file
PLA-1823 Rename permission name to restricted_promote per CR comment
Merge pull request #4907 from Wikia/PLA-1823

PLA-1823 Leave Special:Promote for staff only