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/reports/show.html.haml

@@ -68,9 +68,11 @@
 
 %hr.spacer
 
-%div{ style: 'overflow: hidden; margin-bottom: 20px; clear: both' }
+%div.action-buttons
+  %div
+
   - if @report.unresolved?
-    %div{ style: 'float: right' }
+    %div
       - if @report.target_account.local?
         = link_to t('admin.accounts.warn'), new_admin_account_action_path(@report.target_account_id, type: 'none', report_id: @report.id), class: 'button'
         = link_to t('admin.accounts.disable'), new_admin_account_action_path(@report.target_account_id, type: 'disable', report_id: @report.id), class: 'button button--destructive'