Allow Local Image Upload

[titaniumbones]

It should be possible to upload local images linked in canvas html export & modify links to point to them. The links should have the format {baseurl}/courses/{courseid}/files/{fileid}/preview, which I already know how to get from my file upload in reveal-wim.

The problem here is that images will be reuploaded repeatedly. So, ideally, we could instead store a relationship between local files and urls in a hash table. Before upload, do a check to see (a) whether image has an entry in hashtable (b) whether the courseid is current.

• [ ] implement file upload option for local image files. This means rewriting the HTML link handler
• [ ] implement hash table as a defcustom or similar.