This is a follow-up from {T218265}.
=== Value proposition
We should support muting notifications for users from //Special:Mute//, which is the special page we will be using for {T218270} and {T218266}. This is also a step forward towards bringing the mute lists closer.
=== Acceptance criteria:
* Add the second checkbox (//Mute notifications from this user//) on Special:Mute page:
{F28582602, width=400}
* If a user checks this checkbox and hits save, the user they are muting is added to the //Muted users// list under the //Notifications// tab in Special:Preferences.
* After save, the checkboxes and //Mute user// button go away and a confirmation message displays: //User:ABC has been muted. See all muted users in [Special:Preferences](https://en.wikipedia.org/wiki/Special:Preferences)//. - the link goes to Special:Preferences on the wiki they are on.
* If a user lands on //Special:Mute// with the user they request to mute already being in one or both of the mute lists, those respective checkboxes are already checked. Unchecking a box and saving the form, removes the user from the respective mute list.
!!Note that this only holds if Echo is installed and enabled. If there is no Echo, we only show the one checkbox that's added in T218265!!