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.

since everything (except the CurrentKeyChain method) operates on a
tempoary keychain anyway we don't need this anymore.

According to https://github.com/twbs/bootstrap/issues/10547 it's not
possible to have the trigger defined on the delegated element, i.e. not
defined as a data attribute.