Convert network initialization queries to sqlalchemy and associated changes

[ProbablyFaiz]

REQUIRES MIGRATION

Note: due to foreign key constraints, you may need to run the migration with the flag data=true as follows: alembic upgrade -x data=true head

This PR's main purpose is to convert the queries that back the network's initialization to use sqlalchemy instead of peewee. It also does a whole bunch of infrastructure work to enable both that and future use of sqlalchemy, including:

• Marks appropriate columns as foreign keys and adds migrations to enforce these foreign keys on the database level.
• Adds relationships to the models so that related resources can be easily accessed through sqlalchemy queries.
• Adds model helper methods to Citation to make it easier to filter citations by citing and cited courts.
• Changes to the alembic revision template to make data modifications in migrations (a) possible and (b) optional via a CLI option.

Closes #31.

[varun-magesh]

lgtm, just tested + works