sync plist rep of assignment from org-mode to what we get from canvas request

[titaniumbones]

As much as possible, make the data we get from an Assignments.org file sync with what we get when we ask canvas LMS for a list of assignments.

Then use this uniform data structure to move Assignment data around from place to place.

Along the way, create some helper functions to do the heavy lifting around e.g. property creation. Eventually this should lead to using org-ml to build a data structure, and hopefully eventually this can all be framed out to an async process, in some distant future.