Upload Download Kernel Images

[Winston-Hsiao]

What does this PR do?

Allows admins and moderators to upload Kernel Images and allows for users to download the Kernel Images from the downloads page.

[Winston-Hsiao]

@codekansas

main highlights on current approach/implementation:

1. prev artifacts/upload/{listing_id} api route now artifacts/listing-upload/{listing_id} as they're tied to listings
2. new independent artifact upload now utilizes artifacts/upload
3. new fields to Artifact model:

• label with type ArtifactLabel for distinguishing between kernel image or ml weights/whatever it is we're trying to store ML wise (it was unclear from the different requests from people)
• downloads int for tracking number of downloads we'll atomically increment that on download by signed user
• is_official boolean

4. Downloads page should fetch Artifacts with label "kernel" or "ml" as those are the 2 artifact types we want downloadable from this page (JX made the decision to drop URDFs from this page and have those continue to be primarily accessed via /browse)
5. Downloads page has a button visible only to authenticated users with is_mod and is_admin permissions, and independent upload process also validates current_user has permission to upload to s3

Notes/concerns: not sure if the new independent artifact upload flow I've made is optimal or if it's not fully backwards compatible with the onshape/listing artifact flows. CRUD functionality is working but not sure about correctness with S3/cloudfront and the actual files being uploaded/to be download.

[Winston-Hsiao]

Notes: Had to encode/decode file as Base64 string to get rid of linting/typesafe error in Download.tsx that would cause build to fail. Passing file as raw binary worked and was functionally correct, however the Kernel Image router upload endpoint expects file as a string, and form data and other variations with different request headers would not get rid of the type "string" requirement.

Can return to this to remove base64 encode/decode and pass as raw binary (save space and time) during data transfer.