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