Hide the clue editor behind a gandalf bridge

Summary: Depends on D7421

Test Plan:
1. Load perseus editor in local mode, add a multiple choice widget, verify no JS errors
2. subperseus on top of D7421
3. Create a new gandalf bridge at /gandalf called "allow_editing_clues"
4. Load an item w/ multiple choice via /devadmin/content/items; verify the clue editor isn't visible
5. Load an item w/ multiple choice via Perseus One; verify the clue editor isn't visible
6. Create a gandalf rule matching my account
7. Load an item w/ multiple choice via /devadmin/content/items; verify the clue editor IS visible
8. Load an item w/ multiple choice via Perseus One; verify the clue editor IS visible

Reviewers: brianmerlob

Reviewed By: brianmerlob

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7422