Create TinyMCE Media plugin with Image editing

Eventually this should be a single plugin which adds:

• Insert Image
• Insert Video
• Drag/Drop handling
• Media management

See https://www.tiny.cloud/docs/tinymce/6/upload-images/ for information on drag/drop uploading within TinyMCE.
We should find a way to disable the native drag/drop handling when the editor does not support file uploads at all.

This should integrate with the Repository API.

Decision to be made on whether our custom plugins should use the Moodle dialogues, or the TinyMCE ones.

In this issue

This issue is all about getting the image insertion working. That is:

• drag/drop upload
• replicating the current TinyMCE standard / Atto standard image features into a Moodle modal
• ideally displaying a preview

If context menus are easy, then they can be done here too, otherwise we can move that to a separate issue.

Note: The epic contains a start on the above to get the drag/drop working and to add a fake implementation of the button.