Define FK relationship between Entry and MorphTypeData

[myieye]

We migrated Entry.MorphType without syncing MorphTypeData, so we need users to sync MorphTypeData before we add a FK. (Well, we'll at least give them a few weeks before we define it?)

A FK would let us more efficiently get the current name and syntax/punctuation.
The enum is theoretically helpful, but not sufficient.
We've put the enum on the Entry, so we probably just use that as the FK? Then MorphTypeData.Id is essentially unnecessary, but oh well.