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/filters/index.html.haml

@@ -5,8 +5,7 @@
   = link_to t('filters.new.title'), new_filter_path, class: 'button'
 
 - if @filters.count == 0
-  %div{ style: 'display: flex; justify-content: center' }
-    %div.muted-hint= t 'filters.index.empty'
+  %div.muted-hint.center-text= t 'filters.index.empty'
 - else
   .table-wrapper
     %table.table