This is not part of any designs, so it may be short-lived, but it's quite handy
for diagnosing issues with cross-signing at least while the feature is in
development.
And also a bunch of other UI fixes in the devices table:
* It's the devices table, don't need 'device' in all the headers
* Not really necessary to label checkboxes with 'select'
* Stop table from moving down when the delete button appears
Fixes https://github.com/vector-im/riot-web/issues/10216
