Introduce directionality to mappings

[jsstevenson]

Two broad goals

1. Some or all mappings should be directed -- at the very least, translation and transcription
2. It should be easy to both write and retrieve mappings in a way that supports lookup from both directions. When I put a mapping, there should be a way for me to write it in both directions in one pass. Similarly, I should, for example, be able to look up everything that translates to a variant, as well as translates from a variant.

The current state

1. A Mapping consists of a source_id, a dest<ination>_id, and a MappingType, where MappingType can be "liftover", "transcription", and "translation". (i.e. undirected)
2. The AnyVar .get_mappings() method takes a source_id as an arg, and retrieves all mappings where Mapping.source_id matches that. Which means...
3. ...if you want to support lookup in both directions of a Mapping, you need to write that mapping twice, flipping source and destination. And directionality of the relationship is not preserved.

A few stray ideas (maybe mix and match from these)

1. We change the MappingType enum values to transcribes_to and translates_to, and change AnyVar.get_mappings() to accept args for source_id, dest_id (+ maybe an "either" operator?)
2. We change the MappingType enum to include transcribes_from, transcribes_to, translates_from, and translates_to. When appropriate, or at the request of the user, the AnyVar.put_mapping() method should write both directions at once so a user doesn't have to call it twice