Allow creating posts that are not sourced from DeviantArt

[IsaacSchemm]

This could include three types of posts:

• artwork posts (with a title and at least one image)
• journal entries (with a title, federated as Article)
• status updates (with no title, and zero or more images)

You should be able to crosspost these posts to Bluesky, Weasyl, and/or DeviantArt as appropriate, and they should be included in the ActivityPub outbox.

This and #33 together would make DeviantArt roughly symmetrical with how Weasyl and Bluesky are supported (with the exception of backloading from dA, which isn't supported for the other two).

[IsaacSchemm]

If UserPosts can be migrated to this new type (while keeping the same ActivityPub ID) then I can remove the DeviantArt import functionality (and reimplement it later, if I need it later). Maybe this would be a 4.x branch, with UserPost as an obsolete object whose properties get copied over.