CLC-3594: NameError in MyActivitiesController#get_feed

https://jira.ets.berkeley.edu/jira/browse/CLC-3594

As Chris pointed out, this was happening because
app/models/my_activities/canvas.rb had the same name
as Canvas the module.

I renamed myactivities/canvas.rb to myactivities/canvas_activities.rb
I also renamed the class to MyActivities::CanvasActivities,
and made CanvasActivities a class inside the module MyActivities,
which is the proper way of doing it.

Confirmed that the NameError in MyActivitiesController#get_feed
no longer appears.