[WIP] 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.
some of these need a `terms` query rather than a `term` query for those fields we stick a list in ES. [See here](https://github.com/mozilla/zamboni/pull/2476/files#diff-3fe5de2dc60e34dd922388f3d0625b4dL149)