* 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
