2714 filter by confirmation date

[iamleeg]

Added the first filter terms (the only field in the schema at the moment is confirmation date, so the filter is for confirmation date). Some design notes:

1. The controller parses the filter terms from the HTTP query and builds some technology-agnostic predicates.
2. The mongo store interprets the predicates to build a mongo query. It does this by adding a to_mongo_query() method to each type of filter. You could imagine that if we added a SQL store, it could add its own to_sql_query() methods allowing it to build a WHERE clause.
3. Keywords used to construct the filter, i.e. dateconfirmedbefore/dateconfirmedafter are the same as the existing data service.
4. On a large-scale data service you would want an index on confirmation_date for these searches to be efficient. I'm not doing indexes yet because there will be a lot of flux until the Day Zero schema is defined (and other decisions, like whether we use the list:true flag or some other mechanism for pseudo-atomic updates, are made). Currently a good option looks like pymong-migrate which could be embedded into this service so that on launch it migrates any indexes/validators it needs: open for discussion though!