Allow administrators to configure global default settings

Summary:
Ref T4103. This just adds a single global default setting group, not full profiles.

Primarily, I'm not sure how administrators are supposed to set profiles for users, since most ways user accounts get created don't really support setting roles.. When we figure that out, it should be reasonably easy to extend this. There also isn't much of a need for this now, since pretty much everyone just wants to turn off mail.

Test Plan:

• Edited personal settings.
• Edited global settings.
• Edited a bot's settings.
• Tried to edit some other user's settings.
• Saw defaults change appropriately as I edited global and personal settings.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16048