Provide a wrapper around character conversions

Summary:

• Centralize the check for the "mbstring" extension.
• Centralize the check for "user explicitly specified encoding as utf-8 so just no-op".

Test Plan: Test coverage.

Reviewers: davidreuss, vrana, btrahan

Reviewed By: davidreuss

CC: aran

Maniphest Tasks: T452

Differential Revision: https://secure.phabricator.com/D3252