"DS assignment" option

[hoffm386]

Requesting an overall option that automatically sets:

1. The HTML contents to be just the header (depends on "Header only" option #12 )
2. The assignment payload to be worth 0 points and have a submission type of external tool (depends on Customize payload params #13 )
3. The external tool submission to have the appropriate IllumiDesk link (depends on IllumiDesk submission type option #14 )

This is only applicable for assignments (not pages) since for now, pages in DS work the same as in SE.

This tooling would allow DS to make better use of the other improvements to this gem (e.g. syntax highlighting, solution button) without investing a large amount of time in manually updating Canvas

Thanks!