Fix users and tags sorting

Summary:
- Use ResultsCollection + FilteredCollection mixins in  Set, Tag and User Collections
- Move posts sorting options into PostCollection.sortKeys
- Add 'order' options to <collection>.sortKeys, use this to override order as well as sortkey
- Fix sorting on collections using 'client' paging (Tags Collection)

Fixes T287

Test Plan:
- Check tags sorting
- Check users sorting
- Check messages sorting
- Check posts sorting

Reviewers: dkobia, eyedol, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T287

Differential Revision: https://phabricator.ushahidi.com/D188