Add mute preferences link to the tools menu

Add mute preferences link to the tools menu

A link to Special:Mute is added to the tools menu on the sidebar
if the user is logged in and a registered relevant-user* is found
Mute preferences link will not be shown if $wgEnableSpecialMute is false

* From Skin::getRelevantUser: Special pages like Special:Contributions mark
  the user which  they are relevant to so that things like the toolbox can
  display the information they usually are only able to display on a
  user's userpage and talkpage.

Bug: T218270
Change-Id: I89eb8373963d28d52a026b60aa42d1c64b31b7c6