Improve support for distribution of course material

[damianhxy]

Email from Iliano

in the past, we've mainly used Autolab to distribute programming assignments and have it autograde the students' submissions.

This semester (for various reasons), we also had to use it to distribute various type of course material that is not autograded. Autolab does the job, but it's clunky and students don't access the resources as much as they should. Consider the attached screenshot: each entry provides just a PDF file. It would be great if the students could download this file by simply clicking on it, rather than also going to "options" and then "view writeup".

Plan

• Add a "category" option for attachments, update UI to group them by category
• Add a "release date", superseding the current "released" checkbox

[damianhxy]

To follow up in F23

Related to #1788

[damianhxy]

Note: In the future, would be good to automate attachment creation. e.g. by providing a way to upload from CLI.

The endpoint should return the attachment URL, so that instructors can easily update their course website with direct links.