Import Course Improvements

[damianhxy]

• Allow instructors to overwrite existing course, so that school can create course for them (maybe: check that there are no submissions)
• Ignore useless fields: semester, key (name), date
• When importing, interstitial page to configure fields
• Maybe: set date of one assessment, to offset the rest

122 (updated Apr 15 2024)

• Add an import / export settings button at the top of edit course page (could consider reusing the logic for course yml files)