New feature: folders to organize activities

[dqnykamp]

This PR merges numerous PRs made to the folders branch into main. These PRs add the ability to organize activities into folders. Although the folder feature is still in progress, it now has all the features of the old system.

To make folder function, a number of significant changes were made:

• Assignments and activities are now merged into a single content (and single database table). Activities become assigned, and which point they could be called "assigned activities" or, equivalently, "assignments".
• The Portfolio tab is renamed Activities and includes the assignments from the Assignments tab where the user was the instructor.
• Activities in the Activities tab can be organized into folder.
• Public and private activities are no longer separated but the status is designed on the card.
• When an activity is assigned (i.e., becomes an assignment), clicking its card now shows the assignment view where it can be opened and closed and data can be viewed.
• A "See scores" button on each folder shows data for all assignments in that folder and its subfolders and all students who have taken one of those assignments.
• One can drill down to see data for a single student and all assignments in that folder and its subfolders, as well as drill down to see data for a single assignment.
• A new "Assigned" tab shows all the activities that have been assigned to the student (which currently means there exists assignment scores for that student).
• As we only have quick assign activities for now, the Assigned tab is not organized into folders.
• A "See scores" button in the Assigned tab shows scores for that student on all assigned activities.

This PR also includes:

• a responsive navbar that is mobile-friendly
• a re-implementation of admin features to manage promoted content