Add Special:Mute as a shortcut for muting notifications

 - Special:Mute has been added as a quick way for users to block unwanted
   emails from other users originating from Special:EmailUser
 - Special:Mute can be enabled by setting $wgEnableSpecialMute = true.
   This flag default value is `false`. This flag is temporary until
   enough wikis have this feature enabled and then it will go away
 - When Special:Mute is enabled, emails sent from Special:EmailUser
   contain a link to Special:Mute to facilitate access to the page

In the future, Special:Mute will support extensions to append other types
of notifications that could be muted. These are some of the tasks tracking
the rest of the work:
- T218270
- T220163
- T218266

It is worth mentioning that blocking emails from users is already available
via Special:Preferences

Bug: T218265
Change-Id: I54b847192f42ee1f37999d36c3a187f8826f55a8