Better behavior for missing courses

This is all stuff re-added from #24
pep8 linter cleanup
Add requisite model
pep8 linter cleanup
add enrollment requisite model
pep8 linting cleanup
scrape requisites woo
linting
add missing import
remove erroneous return statement to actually save requisites
don't hard-code links to requisites
link requisites
yeah links and stuff red for non-existent courses
remove debug prints and add a bit of documentation
Merge remote-tracking branch 'origin/develop' into requisites
Merge pull request #73 from ChrisCooper/requisites

Requisites
Merge branch 'develop' of https://github.com/ChrisCooper/QcumberD into develop

Conflicts:
	course_catalog/models.py