Make wallet log context normal

[huumn]

Currently we store an anything-goes JSON object as the wallet log context. We shouldn't:

1. the context object is undocumented (ie not even documented in code in one place)
2. migrating the context object over time (which is inevitable) is going to be messy
3. its keys need to iterated over/filtered in JS to tell if the wallet log has utility in whatever function is using wallet logs
4. context keys can't be properly indexed in the database without making a bigger mess
5. given the above, it's not worth developing views based on the wallet log's relationship to any data stored in context (e.g. viewing all the wallet logs related to an invoice payment)
6. people are going to store random denormalized data in the context object over time because its the path of least resistance for them and pushes the burden of their decisions into the future

Keeping vanity data in JSON blobs is fine, but we shouldn't be storing important data this way. The DB is the root of sanity in our architecture. Storing normalized data in it means we can access data declaratively and can mostly liberate ourselves from typechecking most of our code and writing tests for what/how data is being stored.

[ekzyis]

Something to consider here is that we can only enforce relations for the receive logs since the send logs are stored on the client.

[huumn]

Oh good point. I'm imagining we kind of loosely enforce the same data shape on the client as we do on the server like we do with configs.

[ekzyis]

This was fixed by #1826 afaict