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.

the rugged versions up to 0.26.0b3 had a bug concerning the signature
extraction. The extracted signature was not always the same, probably
due to a buffer (overflow) issue in libgit.

see https://github.com/libgit2/rugged/issues/608

we introduced memoizing, so it's safe to call the method multiple times.

we need to store the keyid to be able to update the signature later in
case the missing key is added later.