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/settings/applications/index.html.haml

@@ -1,20 +1,25 @@
 - content_for :page_title do
   = t('doorkeeper.applications.index.title')
 
-.table-wrapper
-  %table.table
-    %thead
-      %tr
-        %th= t('doorkeeper.applications.index.application')
-        %th= t('doorkeeper.applications.index.scopes')
-        %th
-    %tbody
-      - @applications.each do |application|
+- content_for :heading_actions do
+  = link_to t('doorkeeper.applications.index.new'), new_settings_application_path, class: 'button'
+
+- if @applications.count == 0
+  %div.muted-hint.center-text=t 'doorkeeper.applications.index.empty'
+- else
+  .table-wrapper
+    %table.table
+      %thead
         %tr
-          %td= link_to application.name, settings_application_path(application)
-          %th= application.scopes
-          %td
-            = table_link_to 'times', t('doorkeeper.applications.index.delete'), settings_application_path(application), method: :delete, data: { confirm: t('doorkeeper.applications.confirmations.destroy') }
+          %th= t('doorkeeper.applications.index.application')
+          %th= t('doorkeeper.applications.index.scopes')
+          %th
+      %tbody
+        - @applications.each do |application|
+          %tr
+            %td= link_to application.name, settings_application_path(application)
+            %th= application.scopes
+            %td
+              = table_link_to 'times', t('doorkeeper.applications.index.delete'), settings_application_path(application), method: :delete, data: { confirm: t('doorkeeper.applications.confirmations.destroy') }
 
 = paginate @applications
-= link_to t('doorkeeper.applications.index.new'), new_settings_application_path, class: 'button'