Add internal API docs

[zachlipton]

The docs that are in covid-api-specs are somewhat out of date.

I was thinking of using flasgger (or something else—have only done minimal research, but it seemed more lightweight than converting everything over to connexion or something) to generate some API docs out of docstrings in the code, especially so others can see how to push edits if needed.

[zachlipton]

I came up with a convoluted yet seemingly effective way to programmatically turn the sqlalchemy field list into OpenAPI docs so we don't need to maintain all that every time we want to add/change a field. It's going to take some time to flesh out all the API docs though.