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/accounts/_account.html.haml

@@ -2,7 +2,7 @@
   %td
     = admin_account_link_to(account)
   %td
-    %div{ style: 'margin: -2px 0' }= account_badge(account, all: true)
+    %div.account-badges= account_badge(account, all: true)
   %td
     - if account.user_current_sign_in_ip
       %samp.ellipsized-ip{ title: account.user_current_sign_in_ip }= account.user_current_sign_in_ip