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'