Feature/exercise intro

This PR polishes up the intro/review text behavior per [HLAB-57](https://jira.huit.harvard.edu/browse/HLAB-57).

Examples for testing:
- [http://localhost:8000/lab/exercise/1](http://localhost:8000/lab/exercise/1) (has short intro and review text)
- [http://localhost:8000/lab/exercise/2](http://localhost:8000/lab/exercise/2) (has long intro text, but no review text)

@MichaelDHilborn-Harvard @bilbe Can you review this behavior?