[knowledge-base] Add Contributor Upload Form Backend

[yizou275]

This PR implements the backend of knowledge base contributor's upload page, where they upload the md file of the guide.
I not only include the initial upload page, but also a reupload page, for users whose guide is under "change requested" status and wishes to reupload their guide to address the changes.

Description

The implemented feature of both upload and reupload pages includes the following:

1. Display a form, where users can select a game and upload their file (In reupload page, user can NOT choose a game.)
2. File restriction: if they upload a non-md file, after they hit submit, the page shows a validation error
3. Page access:
   - for upload page, all logged-in users can access this page, and unlogged-in users are redirected to the log in page
   - for reupload page, only logged-in user who authors this guide and is under "change requested" status can access, otherwise will display 403.

File Changes

Specifically, the files I change are:

• views.py: add the ContributorMdUpload view that handles both upload and reupload according to the logic above
• forms.py: add the form that takes in a game and a md file; the game field is read-only for reupload, and non-md file type will throw an error
• urls.py: add the link for upload and reupload
• contributor_upload.html: modify according to views.py to give an unstylished view of the upload page

Testing

To test it, first pull this branch, load the fixture according to knowledge-base/fixtures/README.md, and then runserver.

1. First go to upload page as unlogged-in user, check whether they ask you to log in. If so, log in as Admin.
2. Then check whether the page is displayed properly, including both game and file file.

3. Try upload a random non-md file. Check whether it throws an error. 4. Upload a md file, submit the form. Go to admin interface and check whether there is a new Guide object, with matching author, content, and status=pending. Also, pls record the content for Guide 6 and confirm status=change requested, for later purpose.

5. Then test reupload page. When back, log in as Sam. Go to http://127.0.0.1:8000/knowledge-base/1/reupload, it should throw a 403 (guide author doesn't match); go to http://127.0.0.1:8000/knowledge-base/7/reupload, should throw 403 error (status is NOT requested change). Then go to http://127.0.0.1:8000/knowledge-base/6/reupload, you should then be able to see it displayed, with game field being read-only.

6. To check whether reupload works correctly, reupload and submit another md file. Then log into Admin interface to check what is saved Guide 6 right now -- check whether content is updated and status is pending again.

AI reference: Consulted ChatGPT on how a site can take in a file from a form and render it into our stored data object. Understood and then modified its example code to accommodate our need.

[mrollinswaterman]

All functionality looks good to me! Couple notes on mostly cosmetic/QoL stuff:

• Might be nice to have something to indicate a file has been uploaded properly. Right now, there's no change on the screen so it's kind of hard to tell.

• Hand in hand with that, I think /upload and /reupload should redirect to some other page once the a file has successfully been uploaded. Probably just back to the contributor page, but it's up to you.

Other than that, everything looks great! Good stuff.

[yizou275]

Thank you Miles for this quick and helpful feedback!
Regarding the page after upload, I initially plan that it will redirect to the landing page of knowledge base. This works successfully on my end, but not on Miles' end. As time is limited and this does not affect the correctness of backend update of the database, we agree to merge this version for now, and open a future issue for fixing this bug.

[jacqsmei]

nice work! you can make a follow up pr for showing the upload state (maybe some kind of green check if successful, some error handling if not?)

[jacqsmei]

Issue Score: Excellent

Comments:
Nice work & really thorough testing instructions! And great teamwork in the review