Make the browse button on the tokenizer, be on the tokenizer

Summary: Moves the Browse... button into a Search Icon on the actual tokenizer. I played with a number of icon treatments, and Search seems to convey the right attribute, other things like lists and menus didn't quite feel right to me, but feel free to push back if you hate search.

Test Plan:
Tested lots of tokens, little tokens, small screens, etc.

{F375467}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12452