Allow building of database indexes in the background

With larger deployments coming, we'll certainly need to create a bunch of indices on existing massive tables.

Maybe we should start thinking about dealing with constructing those indices at runtime, maybe not part of the migration? Synapse has something similar, we might want to take inspiration from it

…or we could just `CREATE INDEX CONCURRENTLY` from the migration and call it a day (and ask people to run migrations before rolling out)