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.

config/locales/en.yml

@@ -339,6 +339,7 @@ en:
       delete: Delete
       destroyed_msg: Successfully deleted e-mail domain from blacklist
       domain: Domain
+      empty: No e-mail domains currenly blacklisted.
       new:
         create: Add domain
         title: New e-mail blacklist entry