certify: get rid of fingerprint check

Master key ids are unambiguous by database design, and only keys which
are in the database are signed. There is no reason to check the
fingerprint again during the certify operation.
implement preliminary MultiCertifyActivity (most heavy lifting is done)
Merge branch 'jacobshack-certify' of github.com:open-keychain/open-keychain into jacobshack-certify