feature/rename-add-course

This PR renames the "Add a Course" button to "Add Collection" because it doesn't make sense in Canvas. 

When you "Add a Course," you're really just adding a new collection, which is defined as a set of decks grouped together. An instructor can add as many collections as they want, so it no longer makes sense to maintain the 1:1 course:collection terminology. There was some discussion about the exact button text to make it clearer to users who may not be familiar with the collection > deck > card hierarchy that our tool uses. 

@jazahn Review?

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[FLASH-152](https://jira.huit.harvard.edu/browse/FLASH-152)