when omitting the end part of the key ('-----END PGP PUBLIC KEY
BLOCK-----') the error message was not about the key anymore, but about
the missing fingerprint and primary_keyid, which was confusing for the
user.
the new validation checks that the end also matches the expected format.

we actually don't need a limit on the column itself for MySQL to work.
we need to set a length on the index.

also:

* reorder table columns
* no need for `add_concurrent_index`
* no need for explicit index removal on `#down`

other than just removing a key, which doesn't affect the verified state
of a commit, revoking a key unverifies all signed commits.