Use a single list of attribute entity types

This was previously scattered throughout the code and was susceptible to
getting out of sync.
Add virtual post-processing method for uploadables
Post process subsidy info for attribute values

Populate the attribute value table with unique values from the newly
imported
subsidy information
Convert helper methods for AbstractUploadable
Add virtual PreProcess method

Make changes to the rows before import or no-op
Pre/Post process project values for filter lists
Cache attributes call

Add more aggressive caching to attribute/value requests.  Invalidate
cache
when new data is uploaded.
Merge pull request #41 from azavea/topic/attribute-caching

Cache attributes call
Merge pull request #40 from azavea/topic/project-post

Pre/Post process project values for filter lists
Merge pull request #39 from azavea/topic/subsidy-post

Extract subsidy lookup values for filter after upload