Followers-only post federation (#2111)

* Make private toots get PuSHed to subscription URLs that belong to domains where you have approved followers

* Authorized followers controller, stub for bulk action

* Soft block in the background

* Add simple test for new controller

* Rename Settings::FollowersController to Settings::FollowerDomainsController, paginate results,
rename "private" post setting to "followers-only", fix pagination style, improve post privacy
preferences style, improve warning style

* Extract compose form warnings into own container, show warning when posting to followers-only with unlocked account

app/assets/javascripts/components/locales/en.jsx

@@ -99,7 +99,7 @@ const en = {
   "privacy.direct.long": "Post to mentioned users only",
   "privacy.direct.short": "Direct",
   "privacy.private.long": "Post to followers only",
-  "privacy.private.short": "Private",
+  "privacy.private.short": "Followers-only",
   "privacy.public.long": "Post to public timelines",
   "privacy.public.short": "Public",
   "privacy.unlisted.long": "Do not show in public timelines",