Module model answer RevealRule is not taken into account in CachedContent

[lainets]

The RevealRule is checked and saved in CachedPoints, which is used on course pages to check if the model chapter should be accessible. However, CachedContent does not check the RevealRule, meaning that the chapter could possibly be accessed somewhere using CachedContent without knowing that it shouldn't be accessible.

Investigating whether this truly is an issue (and fix it if there is one) would technically be enough, but it would be preferable (and easier) to implement is_revealed into the CachedContent such that it is false for all chapters that are model answers to a module. This should making such a mistake in the future less likely. After the cache changes, it would be easy to add a @property that checks if the model_answer_modules list is empty.

Highly related to #1242, as this could be done as part of that refactoring.