Make lease choices a GraphQL enum

[toolness]

When working on #138 and #139, I noticed that the mail choices actually make their way to globalTypes.ts as an enum, unlike our other choices. I think this is because we expose the LetterRequest model to GraphQL via Graphene's DjangoObjectType class, which I've never used before, and I think that contrary to the other class we've been using, DjangoFormMutation, it actually represents choices as GraphQL enums.

It would be nice if we could do this with the lease choices in particular, as we hard-code them in onboarding step 3.

Update: It would also be helpful if borough choices were a GraphQL enum too; I had to manually define them as a TS enum in #192.