Refactor exercise_accessible and is_revealed #1242

The exercise_accessible tag should be refactored to work similarly to the is_listed and is_visible tags (or later methods with the new cache changes done in PR #1231). This means saving whether it is accessible to the cache (or saving enough data to the cache to easily compute whether it is accessible) and checking that instead of computing the value in the templatetag file exercise/templatetags/exercise.py. As part of this refactoring, the is_revealed attribute (in CachedPoints) should be taken into account in whether it is accessible, and the is_revealed checks can be removed from the HTML files.