Improve course creation page

[geovgy]

The creation page for new courses is limited to uploading an ".xlsx" file with a single sheet. The course items must be laid out in specific cells to upload correctly. To improve on this, looking for two things:

1. Fix the upload function to allow multiple courses to be uploaded at once.
2. Better error handling in case of a value missing in one of the requested cells.
3. Build other possible options for admins to be able to create new courses besides only ".xlsx" upload. (Possibly add ability to connect directly to a Google Sheet, etc.)

[geovgy]

Progress on this issue can be followed on feat/course-creation branch.

So far, I have improved the UI feedback once a course has been uploaded and ensured multiple courses can be uploaded at once as long as each course is in their own sheet within the .xslx file. The UI feedback will tell the admin which courses have been uploaded by their title.

(2) and (3) are still in progress.