Merge branch 'sshkey_form_feedback' into master

This brings back in a change that was implemented in a pull request
from 2017, but got lost in the chaos of multiple pull requests from
intermingling branches.

This does not only provide feedback when a SSH key cannot be parsed,
but it also does so in a way that the warning goes away when a correct
key is added. Admittedly, I have no idea how to properly do this with
a Wicket FeedbackMessage, all I could find on Google was highly
complicated.

Not only does this bring back (or really in) the fix for issue #1226,
but it also fixes #984.