Add topic to UI (search) + popup

[birchamp]

In the search bar users should be able to specify the 'topic' field along with other query fields. The popup should be updated to reflect this new functionality. Clicking on a topic should use that same search logic.

DoD:

Searching "topic:xxx, lang:xx" will result in a filtered list of tagged repos in that language. The popup should correctly describe this. Clicking on a topic label should produce a URL that is the same format as searching for topic.