Folders UI

Initial PR to get a sanity check on the work done so far.

The following items have not been tackled yet:

- Changing the visibility of a folder
- Deleting folder(s) from the system
- Adding existing items to a folder
- Change the URL when switching between the content and folders tabs
- A large part of the tests
I'll remove `aboutdiscussion` and `aboutgroup` from my PR in favor of the test refactor PR.

After I wrote these tests, I did a structural review for the test refactor PR and have held off writing more tests because of the potential for significant merge conflicts. It would be good if the test refactor follow-up could be done first so that can be merged before I write all of the missing folder tests. It will also make writing tests significantly easier.