Finish implementing search

[Eevee]

• Figure out how to make multiple schemata work in one index
• Figure out the problem of e.g. code needing different tokenizing than prose
• Add a hook (not an event hook surely? what then?) for loaded plugins to expose indexable content types
• Add automatic indexing and reindexing, probably via SQLA decoration
• Figure out some way to automatically reindex everything, maybe via an admin/stats page