Search/filter feature

First version (hence the branch's name) for search feature based on Visual Search.
This has been rebased so many times to catch up with the develop branch.

Specification here: https://docs.google.com/a/hypothes.is/document/d/1mLxjZWAd-G35_DagYv9RcpMBoXUnoXBSQjZh8eodbJQ/edit