Valdiate custom field keys

Summary:
Validates the keys on page load, alerts if there is a problem.

Fixes T3432

Test Plan: Use some good and bad keys, ensure we get the error at the right time.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3432

Differential Revision: https://secure.phabricator.com/D6284