CLC-3410 Seventh batch of proxy reorg: Canvas

https://jira.ets.berkeley.edu/jira/browse/CLC-3410

Move everything Canvas-related from lib/proxies/canvas*.rb into app/models/canvas. Remove "Canvas" prefix from these classes since their module makes clear what they are. Remove "Proxy" postfix from proxy classes since inheritance makes it clear what they are. 

lib/canvas stays where it is for now, but will move soon. 

Also fix a small bloop in naming of MyClasses::Sakai that caused an error in Torquebox (but not in the tests, oddly).