Cache predictor fragment in student dashboard.

Closes #505.
Aggressive caching.

Aggressively cache anything related to grades, assignments or badges on
the student dashboard. Cache invalidation will need more tuning,
ensuring that the cached partials will invalidate when less freqeuntly
edited properties, such as score levels, are edited.

Closes #508.
Closes #507.
Merge pull request #506 from bellotastudios/cache

Aggressive caching.