Upload images from SSE to the project repo

What does this MR do?

This MR adds the ability to upload images from the Static Site Editor (SSE) to the project repo.

This is the 2nd iteration towards delivering #218529 (closed)

This MR excludes previewing of inserted images (addressed in a separate MR)

Note: this MR is behind the sse_image_uploads feature flag

To enable the feature:

1. Run rails c in the project root
2. Run Feature.enable(:sse_image_uploads) in the rails console
3. Refresh the browser

Preview

TODO

• Persist images in a local image repository
• Make the imageRoot configurable on the image modal
• Add inserted image to the MR
• Ensure images are not uploaded if they have been removed from the Markdown content
• Ensure duplicate images aren't uploaded if the user inserts the same image twice