Substantially support character encodings and "Highlight As" in changesets

Summary: Ref T5179. Ref T4045. Ref T832. We can now write non-utf8 hunks into the database, so try to do more reasonable things with them in the UI.

Test Plan: (See screenshots...)

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T832, T4045, T5179

Differential Revision: https://secure.phabricator.com/D9294