add postgres support

[waqasbhatti]

This will involve:

• separating dbsearch.py into two modules: sqlite.py and postgres.py
• adding in support for kwargs for indexserver to pick up which backend to use
• adding support for full-text search in postgres (it doesn't have BM25, but maybe pg_trgrm ops on a GIN index on the appropriate FTS columns is enough? we could also implement BM25 in Python and rerank after results come back)
• searchserver_handlers will have to something like from ..backend import sqlite, postgres instead of from ..backend import dbsearch
• decide what to do with the lcc-datasets.sqlite and lcc-index.sqlite databases we use (I think we can keep them around, no need to shove everything into postgres)

The translation between sqlite -> postgres:

• separate collections go into separate tables instead of separate databases
• we can now join across tables (if we ever wanted to)