Add public API endpoint for public collections

[tw4l]

Fixes #1051

If org with provided slug doesn't exist or no public collections exist for that org, return same 404 response with a detail of "public_profile_not_found" to prevent people from using public endpoint to determine whether an org exists.

Endpoint is GET /api/public-collections/<org-slug> (no auth needed) to avoid collisions with existing org and collection endpoints.

@SuaYoo happy to make any changes needed for the frontend, just lmk!

[tw4l]

Converting to draft while I make changes

[tw4l]

Made all of the changes except the 404 -> 200 change. Also restructured the response a bit to look like:

{
  "org": {
    "name": "Org name",
    // Can easily add other fields here moving forward
  },
  "collections": [
    {"id": "some uuid", "name": "A public collection", "access": "public",...},
    ...
  ]
}

[tw4l]

Also tagged @ikreymer to see if he has thoughts/comments/suggested changes (overall and/or on the 404 question)

[tw4l]

Added support for a public org description (editing via same /public-profile endpoint that's used to enable/disable public profile), as requested in #1051 (comment)

[tw4l]

Some additional changes made:

• Public profiles can now include a URL field
• /public-collections endpoint still returns data if org has public profile enabled even if there are no public collections yet
• 404 message for /public-collections endpoint modified to public_profile_not_found

[SuaYoo]

Looks good! Tested with #2172