Implement Street model

[bertday]

Currently AIS has a single model for street segments, but for implementing a /search endpoint that supports a generic street lookup we'll need a more abstract model called Street that relates all the segs with the same street code. The fields should be:

predir: text (not null)
name: text (not null)
suffix: text (not null)
postdir: text (not null)
full: text (not null, indexed, unique)
code: integer (not null, indexed, unique)

Add a foreign key to StreetSegment called street that references a Street object via its street code. This should be indexed as well. Remove fields in StreetSegment that are covered in the new model (street_predir, street_name, street_suffix, street_postdir, street_code).

Some of the engine scripts will need to be tweaked to use this model, most importantly load_streets, load_dor_parcels, and load_addresses.

[bertday]

Not sure what the use case was for this; closing for now.