The SSH Settings page now uses the modern CSS components, making it fit

in with the rest of the administration UI.
There are a few improvements to the text shown in the UI. We now have a

banner that explains where SSH keys are used (helping to avoid confusion

by users who think they're mandatory).
Form handling has also improved, making use of the add_error() method

on forms to set error details, and making use of submit buttons rather

than multiple form elements to set the action being performed (upload,

delete, generate).

Tested uploading keys, generating keys, and deleting keys.
Tested all the possible error conditions, simulating each one to see

the resulting error information.
Unit tests pass.