Add support for creating multi-issuance CATs

[Rigidity]

Currently when issuing a CAT, you can only create a new single-issuance CAT with the first coin that you're spending as the genesis. There should be the option to create a new multi-issuance CAT, and to increase the supply of an existing one.

This also entails syncing the TAIL of a given CAT and saving it in the database.