Initial DB Domain Types

[gbdubs]

This PR checks in the initial schema for the PACTA app, as described first in the requirements, and then in the Schema Review. It also checks in the first ~10 pieces of sqldb glue code to wire up + test our ability to persist and maniuplate the golang domain types. The first pieces chosen for this are first ones I need to implement the first piece of the site functionality, namely: managing initiative-user membership, which includes creating initiatives, users, invitations, and their acceptance flows.

[bcspragu]

LGTM, though with comments on a few general patterns that I think we should figure out before adding more entities:

• Timestamp stuff (e.g. created_at populated automatically per schema, using CURRENT_TIMESTAMP, etc)
• Using full domain structs for creation methods even when most fields are unused
• exactlyOne and query versus queryRow when querying on primary keys
• Using RunOrContinueTransaction in places with only a single SQL statement