Add attachment support for "Attachment" columns in Airtable

[KalobTaulien]

I noticed recently there's a new "attachment" column type in Airtable. I haven't looked into using it yet, but I suspect we can use it to support images and documents.

Acceptance Criteria:

• Must support images as this is the highest demanded feature right now
• Must support documents as that's part of the "attachment" field.

[RealOrangeOne]

Had a look into this for file-based attachments, and it should be reasonably simple to implement:

• Custom document model, so the Airtable attachment id can be stored for future reference
• Custom serializer to map between Wagtail documents and Airtable attachments:
  • For import, this'll require downloading the file at the specified URL into the document's file
  • For export, just the Attachment ID needs to be returned for existing objects. For new ones, it'll require sending up a public URL for the document (.file.url should be fine)
• Ensure M2M models are correctly updated and diffed when inserting.

Besides the final point, I'm sort of expecting to be able to do this without modifying the library at all, and even that's only a guess. I'd expect something similar to work for images.

Attempting in the next few weeks - I'll report back my findings!