skinnier plugins and themes tables

[chuckadams]

When these were created, I went for the simplest thing to build DB queries on, thus exposing every field in the metadata as its own column, even though it was redundant with the JSON contained in the metadata column. While 42 columns for the plugins table is not that bad, it's still cognitive overhead we don't need to impose whenever we do modify the model or add alternate sources that may have different metadata. Postgres can index a jsonb column pretty aggressively if needed, and if we need more, there's always ElasticSearch and similar.