While porting the FileImporter interface to use Codex, we made several major changes to the user experience. Document these changes and validate the new workflow, making changes if needed.

New behaviors (see subtickets of epic for screenshots and screencasts):

• File title heading can be edited in-place with Edit/Preview toggle button. Previously "Edit title" would redirect to a separate page with the title input.
• File info wikitext can be edited in-place with Edit/Preview toggle, was previously a separate page. Preview sets a gray loading state, then updates the rendered HTML and a list of detected categories.
• "Show changes" injects a diff block into the current page rather than showing a separate page.
• Import action shows a ProgressBar overlay and submits via a background request. Errors are injected into the form as annotations on fields. Success leads to a redirect.