Prevent administrators from locking themselves out of applications

Summary: Ref T603. This could be a nicer UX, but limit the amount of foot-shooting that users can possibly do. You can still manage if you're really tricky ("Members of project X", then leave the project) but this should make it hard to make a mistake. It seems very unlikely any user ever intends to lock themselves out of an application.

Test Plan: Set an application's view policy to permissive ("Administrators") and nonpermissive ("No One") values. The former were accepted, the latter rejected.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7218