Deal with clients sending us dBm signal strength in the asu field.

MozStumbler seems to send a mixture of ASU and dBm units in the asu field.
Might depend on whether the data is for a serving or neighboring cell.
Remove the s3_backup_prefix config setting.

We use per-stack backup_bucket's, so there's no longer a need to encode
the stackname into a prefix.
Add alembic migrations for lat/lon int -> double, cc #267
Change models to use double for lat/lon, cc #267
First pass converting int lat/lon literals to double, cc #267
First cut removing (to|from)_degrees, cc #267
Fix lat/lon bugs in ichnaea/backfill/tests.py cc #267
Change a bunch of ints to floats for lats/lons, cc #267
Attempt to fix some int-float lat/lon issues cc #267
Fix more tests broken by float lat/lon cc #267
Switch Double columns to not use Decimal, cc #267
Switch int division to float division cc #267
Fix more float lat/lon bugs cc #267
Alter mapstat to use floating point coords also cc #267
Fix more float lat/lon bugs cc #267