Use heading actions and placeholders in settings (#12801)

This commit:

- Refactors centered text blocks currently used for placeholders
  for empty tables and puts styles for it in separate class -
  .centered-text, simply aliasing text-align: center. Which is
  furtherly used in this commit.

- Improves applications settings page to use heading actions, moving
  "New application" button there, and displaying placeholder "You
  have no applications" in place of empty table.

- Improves custom emoji settings page to use heading action
  for "Upload" button, making it more easily accessible without
  need to scroll through all of the emojis.

- Improves email domain blocks settings page, moving "Add new" to the
  heading actions and using placeholder "No e-mail domains currently
  blacklisted" instead of showing empty table.

app/views/admin/custom_emojis/index.html.haml

@@ -4,6 +4,9 @@
 - content_for :header_tags do
   = javascript_pack_tag 'admin', integrity: true, async: true, crossorigin: 'anonymous'
 
+- content_for :heading_actions do
+  = link_to t('admin.custom_emojis.upload'), new_admin_custom_emoji_path, class: 'button'
+
 .filters
   .filter-subset
     %strong= t('admin.accounts.location.title')
@@ -81,6 +84,3 @@
 
 = paginate @custom_emojis
 
-%hr.spacer/
-
-= link_to t('admin.custom_emojis.upload'), new_admin_custom_emoji_path, class: 'button'