Implement a rough browse view for tokenizers

Summary: Ref T5750. This adds a basic browse view. Design is a bit rough, see T7841 for some screenshots.

Test Plan: Used browse view to add tokens to tokenizers.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5750

Differential Revision: https://secure.phabricator.com/D12441