Fix admin-facing uses of inline CSS (#13575)

* Move .back-button inline styles to CSS file

All occurrences of the back-button CSS class used the same inline
CSS rules, so moved them over to the CSS file

* Fix “Add new domain block” button using inline CSS

* Replace common pattern of inline-styled button boxes by a CSS class

In particular, switching from `float: left/right` to a flexbox with
`justify-content: space-between`. This implied changing the order of
a few HTML tags and adding an empty `div` in one case.

Also removed a `margin-bottom` rule that wasn't needed due to the
margins of surrounding elements.

* Move account admin view inline CSS to CSS file

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

@@ -10,7 +10,7 @@
       - unless whitelist_mode?
         %li= filter_link_to t('admin.instances.moderation.limited'), limited: '1'
 
-  %div{ style: 'flex: 1 1 auto; text-align: right' }
+  %div.special-action-button
     - if whitelist_mode?
       = link_to t('admin.domain_allows.add_new'), new_admin_domain_allow_path, class: 'button'
     - else