Add possiblity to store key verification

We currently do not store this anywhere. This is needed for any (currently discussed) UI displaying whether we trust a key's ownership.

This commit does not include any fancy UI yet. Verification is only shown in the key verification activities and in the list of our contact's keys. Key verification is done via a MAC over the recipient's ID + the key fingerprint + some tag. The tag is needed to make this MAC different from the "isValidKey" MAC we currently store.