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/show.html.haml

@@ -45,11 +45,11 @@
 
 %hr.spacer/
 
-%div{ style: 'overflow: hidden' }
-  %div{ style: 'float: left' }
+%div.action-buttons
+  %div
     = link_to t('admin.accounts.title'), admin_accounts_path(remote: '1', by_domain: @instance.domain), class: 'button'
 
-  %div{ style: 'float: right' }
+  %div
     - if @domain_allow
       = link_to t('admin.domain_allows.undo'), admin_domain_allow_path(@domain_allow), class: 'button button--destructive', data: { confirm: t('admin.accounts.are_you_sure'), method: :delete }
     - elsif @domain_block