Maintain posts search state via URL

Summary:
- Combine posts list routes and process query params into search params
- Populate the search bar based on current posts collection filter params
- DRYer search handler with deferreds
- Push search query string into URL with navigate()

Fixes T348

Test Plan:
- Go to homepage, search for something
- See the URL change
- Go to a post, go back, get expected search results

Reviewers: eyedol, dkobia, shadowhand

Reviewed By: shadowhand

Subscribers: srutto

Maniphest Tasks: T348

Differential Revision: https://phabricator.ushahidi.com/D198