Respect "can edit username" in registration UI

Summary:
Fixes T3741. The flag is respected in terms of actually creating the account, but the UI is a bit unclear.

This can never occur naturally, but installs can register an event which locks it.

Test Plan:
Artificially locked it, verified I got more reasonable UI;

{F81282}

Reviewers: btrahan, datr

Reviewed By: datr

CC: aran

Maniphest Tasks: T3741

Differential Revision: https://secure.phabricator.com/D7577