dashboard: keys edited, the last removed with a password, failures kept

The credentials dialog listed a user's keys whole, to add one or
remove one but the last, and any failure replaced the dialog with the
error, the form lost.

* A key is shown short: its type, the first and last four characters
  of what tells it apart (an Ed25519 key's base64 after the part all of
  them share), its comment; the whole line in its tooltip.
* A key can be edited, its name and its line: saved under the name, a
  new name written first and the old one removed.
* A user with a password on this node (`--htpasswd`) may remove its
  last key: it logs in by the password. GET /auth/keys says so
  (`password`), and DELETE /auth/keys/{id} takes it then; the rules no
  longer keep the last key on every node (no other node can tell who
  has a password where; under a strict policy the signed entry keeps
  one still).
* What fails says why beside where it was asked (the add form, the
  edited row, the removed one), which stays as it was, to be set right
  and tried again.

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