jabber: Rework jabber_caps_calculate_hash() to allow any size digest

jabber_caps_calculate_hash() previously only allowed hash digests of
up to 20 bytes. There was a note stating to check the size if any
new hashes were used since the cipher API at the time of that writing
didn't include a way to get the size of the hash digest. Now that
it's been ported to GChecksum, which does include such a function,
it can be made to support any size hash digest. This patch does so.