Feature/tlt 466 adding api to new batch job to grab section data from canvas

Adds the django command sync_canvas_sections which will keep the section and section_member tables in the course manager database in sync with the canvas sections. For this command Canvas is the master sections holder, any changes in canvas will be synchronized with the coursemanager database but not the other way around. changes to the course_manager database will not be synced with canvas. 

If a section already exists in the coursemanager it will be updated if the name changed or if there are member to be added or removed. If the section does not exist, it will be created. 
.get() defaults to returning None I think (I didn't know this until doing the code review -- thanks), so I think going forward we can be consistent about not bothering with the second argument if it's going to be None. I will do this too in my code.