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/relationships/index.html.haml

@@ -17,7 +17,7 @@
       %li= filter_link_to t('admin.accounts.location.local'), location: 'local'
       %li= filter_link_to t('admin.accounts.location.remote'), location: 'remote'
 
-  .back-link{ style: 'flex: 1 1 auto; text-align: right' }
+  .back-link
     = link_to admin_account_path(@account.id) do
       = fa_icon 'chevron-left fw'
       = t('admin.statuses.back_to_account')