Add support for extentions to change Special:Mute form

The hook (SpecialMuteModifyFormFields) is used to append
the option to mute/unmute notifications from a specified user.

Special:Mute handles posting and saving the fields, the only
requirement is that the field name is the same as the property
that wants to be modified.

Currently there are only two notifications "blacklists":
* `email-blacklist` is directly handled in this page 'cause it is part of core.
* `echo-notifications-blacklist` is part of Echo, so this change is required
  to support it. See I77b3ccfdce9b501e

Bug: T220163
Change-Id: I2b3eee0802cb086091f35ecce13ae77a8e7d518d