Composer image upload should render a placeholder during upload

[Southclaws]

When inserting an image on a slow network connection (mine lol) there's no placeholder, and you're free to move the caret around resulting in unintended placement of the image.

Some work:

1. don't allow editing during an upload (maybe, depending on the next bit)
2. render a placeholder during upload

There are some considerations too:

• if a placeholder is rendered, and the caret moves, the image itself needs to replace the placeholder rather than be placed at the current ProseMirror caret position
• Error states need to be handled, so if the upload fails, the placeholder needs to be removed

[Southclaws]

Bonus, which I really should do as well, progress bar for upload. Though I'm not sure how possible this is with the fetch API: