CANVAS-114 Refresh Canvas section memberships from campus data

https://jira.media.berkeley.edu/jira/browse/CANVAS-114

This is a big & complicated PR. The end goal is to support the following Rake tasks:

_Updates Canvas section enrollments & teachers from campus data for the current terms._
rake canvas:full_refresh    

_For nervous types, this just generates the files that would be used by the full_refresh task, giving you a chance to inspect and manually import them._
rake canvas:make_csv_files 

_We really care about the sections' SIS IDs, but we need to fill their course's SIS IDs with "something reasonable" for the imports to work. This optional maintenance task decides what "something reasonable" means. Right now it just generates a unique ID, but if it helps with integration, we might make the string more informative._
rake canvas:repair_sis_ids TERM_ID='TERM:2013-C'