Convert user needs form to a bootstrap alert

It was previously a custom CSS alert that was a bit like a
bootstrap alert used elsewhere in the admin.

Also started moving some CSS for the edition form to view CSS.
Fix all alerts in admin being red

Added in c1a50d4f9f0923b1530d9200081753e2ce0a15d1 where there is
some discussion. Alerts which need to be red should use the
`alert-danger` class.