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/views/settings/follower_domains/show.html.haml

@@ -0,0 +1,33 @@
+- content_for :page_title do
+  = t('settings.followers')
+
+= form_tag settings_follower_domains_path, method: :patch, class: 'table-form' do
+  - unless @account.locked?
+    .warning
+      %strong
+        = fa_icon('warning')
+        = t('followers.unlocked_warning_title')
+      = t('followers.unlocked_warning_html', lock_link: link_to(t('followers.lock_link'), settings_profile_url))
+
+  %p= t('followers.explanation_html')
+  %p= t('followers.true_privacy_html')
+
+  %table.table
+    %thead
+      %tr
+        %th
+        %th= t('followers.domain')
+        %th= t('followers.followers_count')
+    %tbody
+      - @domains.each do |domain|
+        %tr
+          %td
+            = check_box_tag 'select[]', domain.domain, false, disabled: !@account.locked? unless domain.domain.nil?
+          %td
+            %samp= domain.domain.presence || Rails.configuration.x.local_domain
+          %td= number_with_delimiter domain.accounts_from_domain
+
+  .action-pagination
+    .actions
+      = button_tag t('followers.purge'), type: :submit, class: 'button', disabled: !@account.locked?
+    = paginate @domains