Reduced index size by not indexing fields we don't query (bug 1055294)

I skipped on the non-string field types b/c integers and booleans don't seem worth the trouble.
I made a helper to spit out the ```{'type': 'string', 'index': 'not_analyzed'}```. We can make one for the ```{'type': 'string', 'index': 'no'}``` too https://github.com/robhudson/zamboni/blob/no-index/mkt/search/indexers.py#L210