* 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

* Add database columns for adding notes to domain blocks/restrctions

* Add admin UI to set private and public comments when blocking a domain

* Add text for private and public comments on domain blocks

* Show domain block comments in admin UI

* Add comments to the domain block undo page

* Make UnblockDomainService more robust regarding upgraded domain blocks

* Allow editing domain blocks

* Rename button from “undo domain block” to “view domain block” in account admin UI

* Change test to unsilence silenced users from upgraded blocks