Ensure Post Image Uploader Reset

Closes #4431

- The PSM does not reset on a transition from editor (existing post) to
editor (new post). If the existing post had a cover image, the image
uploader would not be reset during the transition and appear slightly
broken in the editor for the new post.
- In this PR: A reference to the uploader is saved, allowing the route
for editor/new to instruct the PSM controller to have the uploader
reset.
Merge pull request #4502 from felixrieseberg/iss4431

Ensure Post Image Uploader Reset