Improve rendering of Conpherence fulltext search results

Summary:
Ref T3165. This:

• Fixes a bug with overlapping matches.
• Makes the UI a little less hideous (and more standard).
• Links comments into the chat history view.

Test Plan:

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3165

Differential Revision: https://secure.phabricator.com/D12777