consolidate app filtering for both feed + search

- Combine ```mkt.search.views._filter_search``` and ```mkt.webapps.indexers.from_search``` into ```mkt.webapps.indexers.get_app_filter```.
- Separate the filtering and sorting code.
- SearchView had a lot of needless helper methods, which made the dataflow confusing. I put most of it into one method.