Location lookup table

[skondrashov]

We should be able to perform fast lookup of tweet origin location names with a new table.

The new table should be generated in tweets.sql and filled by a manual script (or at the very least decoupled from archivist's main process) that checks for commonly duplicated lon/lat coordinates and enters metadata for those locations from a sample of tweets in that location.

This process will save space in the database (much fewer location entries than 1 per row) and be backwards-compatible with tweets we have already recorded - we can determine their location names at any time using a lookup against this new table.