Create Cell Area Table (#349)

- Create CellArea table
- Add model tests
- Migrate cell area data from cell table to cell_area
- Change references from Cell table to CellArea table

@hannosch 
Can you change this to explicitly use the dict keys:

``set([CellAreaKey(l['radio'], l['mcc'], l['mnc'], l['lac']) for l in redis_lacs])``

While we do the software update, there still might be unprocessed tasks with the old data (dict of cell keys) in Redis. So when the async workers start up with the new code, they need to be able to tolerate data with the extra cid key. Explicitly only using the four dict keys we care about solves that.