Add API/CLI endpoint for attachments

[damianhxy]

Issues with current implementation of attachments (as per #1983)

• does not support static links (tedious to update existing permalinks)
• no easy way to mass-import

For that reason, using the assessment "direct-download" feature (#2020) is somewhat more convenient for instructors

• only course name changes, easy to update links
• can mass import assessments

Potential solution

• Add an endpoint which uploads an attachment and returns the attachment link -> easy to generate HTML lists
• Rename "attachments" to "course materials"

Also possible: allow the specification of a canonical name

Note: An ad-hoc workaround was available in #2020 #2021 but this would be a longer-term solution.

[damianhxy]

Update as of Apr 15 2024: rather than going down this route, we could simply annotate assessments with a "writeup" label if one exists, and that downloads the writeup upon clicking.