Allow import/export of mutes list (#1541)

* Allow export of mutes list

* Allow importing of mutes list

* Refactor to use Settings::Exports::BaseController and DRY up exports code

config/locales/en.yml

@@ -76,6 +76,7 @@ en:
       x_seconds: "%{count}s"
   exports:
     blocks: You block
+    mutes: You mute
     csv: CSV
     follows: You follow
     storage: Media storage
@@ -92,6 +93,7 @@ en:
     types:
       blocking: Blocking list
       following: Following list
+      muting: Muting list
     upload: Upload
   landing_strip_html: <strong>%{name}</strong> is a user on <strong>%{domain}</strong>. You can follow them or interact with them if you have an account anywhere in the fediverse. If you don't, you can <a href="%{sign_up_path}">sign up here</a>.
   media_attachments: