Oppia export debugging

[bennylope]

• Get Oppia reading the file
• Fix image/resource paths
• Move media files (movie, zip, etc) to media in export
• Fix missing Zip file issue on production

[alexlittle]

On export to Oppia, suggest having a page for users to enter: Oppia publishing endpoint (default to: https://oppia.opendeliver.org/), Oppia username, password and course tags