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/email_domain_blocks/index.html.haml

@@ -1,14 +1,19 @@
 - content_for :page_title do
   = t('admin.email_domain_blocks.title')
 
-.table-wrapper
-  %table.table
-    %thead
-      %tr
-        %th= t('admin.email_domain_blocks.domain')
-        %th
-    %tbody
-      = render @email_domain_blocks
+- content_for :heading_actions do
+  = link_to t('admin.email_domain_blocks.add_new'), new_admin_email_domain_block_path, class: 'button'
+
+- if @email_domain_blocks.count == 0
+  %div.muted-hint.center-text=t 'admin.email_domain_blocks.empty'
+- else
+  .table-wrapper
+    %table.table
+      %thead
+        %tr
+          %th= t('admin.email_domain_blocks.domain')
+          %th
+      %tbody
+        = render @email_domain_blocks
 
 = paginate @email_domain_blocks
-= link_to t('admin.email_domain_blocks.add_new'), new_admin_email_domain_block_path, class: 'button'