Hints are no longer outlined outside Perseus editor

Summary: The CSS selector for outlining hints wasn't quite precise enough and individuals with
Perseus access were seeing hints outlined outside of edit mode. Now, the selector checks that
the hint div immediately follows a hint editor. Fixes T2704.

Test Plan:
- Run Perseus.
- Create a widget + hint and verify that the hint is outlined.
- Run webapp.
- Go to an exercise that was previously outlining hints (e.g., 'Surface and Volume Density Word Problems').
- Verify that hints are no longer outlined.
- Enter Perseus One; verify that hints are outlined in edit more.

Auditors: jack