Post images

[simonv3]

Adds a PostImage table, and when a user drags an image into place it uploads it.

To do:

• Delete images that aren't used anymore
• Let a user see images that are uploaded and mark one as a thumbnail
• Allow a user to click add an image through the toolbar
• tests
  • test that checks that unused images get deleted
  • tests for images
• auto-create a post making it a draft by default

To do with #855

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