[WIP] Editors picks module

This branch moves editors picks out of wagtailsearch and into its own module. This builds on top of my search changes branch (PR #186).

Backwards incompatible changes (these will be fixed):
 - [ ] Wagtailsearch views name changed: wagtailsearch.views.frontend => wagtailsearch.views
 - [ ] Search view no longer adds query object to results
 - [ ] Renamed "search_garbage_collect" to "editorspicks_garbage_collect"

TODO:
 - [x] Create wagtaileditorspicks module
 - [x] Move models into wagtaileditorspicks
 - [x] Move views/tests into wagtaileditorspicks
 - [x] Update url names
 - [x] Check locale strings are working properly
 - [x] Cleanup
 - [ ] Use Wagtail hooks to hook in the admin menu item (blocked by #228)
 - [x] Add template tag for pulling editors picks into search results
 - [ ] Move query object back into wagtailsearch (for backwards compatibility)