Location admin UI should fill some fields automatically

[slinkp]

• last_mod_date should probably be auto_now=True, and not editable
• creation_data should be auto_now_add=True, and not editable
• ~~ slug should probably make use of prepopulated_fields and should probably be a SlugField ~~ Done.
• city - should have choices from our known metros?
• ~~normalized_name - feed Name through ebpub.geocoder.parser.parsing.normalize()? ~~ THis affects multiple models, see Replace ebpub.geocoder.models.GeocoderCache with the normal cacheing framework, and/or purge old cache data #157
• ~~ centroid should be the centroid of the location field (that's what it is in eg. the zipimport script)... in fact i'm not sure if anything ever uses this? a lot of stuff uses some_location.location.centroid instead. ~~ removed!
• area should be computed somehow? (grep turns up code like "area = geom.transform(3395, True).area")

[slinkp]

area fixed.

[slinkp]

(In [1334338]) Remove Location.centroid, we can just use Location.location.centroid instead and not worry about them getting out of sync. Refs #86

[slinkp]

Ticket imported from Trac:
http://developer.openblockproject.org/ticket/86
Reported by: slinkp