Create helper function to remove duplicate code

This repetitive code is especially pronounced in Stanford's fork, where
we have extended the About page to include several additional fields.

This pays down some technical debt by refactoring into a shared helper.
Merge pull request #5874 from stvstnfrd/up/course/about

Create helper function to remove duplicate code
Make changes to the Mongo modulestore format for asset metadata.
Move the asset types under a top-level 'assets' dict to allow for
future arbitrary asset types.
Change 'basename' to 'pathname', since the attr is for storing the path.