CAS support.

[Mihara]

Greetings. We (the Center for Pedagogical Mastery) are currently evaluating RELATE as an alternative to Open edX, (that is, we are hoping to get rid of it as soon as possible, because it's really not a good fit for how we teach, and RELATE appears to be much closer to ideal for us). To do that, the first thing we needed to do is to add support for authorization over CAS protocol, keeping it extensible for the unorthodox uses we've put it for. (We have a separate in-house system managing course subscriptions, which communicates which courses people should be subscribed to as CAS attributes, so the basic attribute mapper that django-cas-ng provides is insufficient.) This, of course, should be generally useful.

To that end, I present this PR, hoping you could accept it. It works, and passes all the tests that were there already, but I couldn't think of what kind of tests would be appropriate to test the new functionality. Should you deem it unsuitable, I'm ready to hack it until it is.

[inducer]

Btw, there were also a few complaints from the linter (Flake8): https://travis-ci.org/inducer/relate/jobs/520386819#L527

Could you look into addressing those, too?

[codecov[bot]]

Codecov Report

Merging #647 into master will decrease coverage by 0.25%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #647      +/-   ##
==========================================
- Coverage   96.93%   96.67%   -0.26%     
==========================================
  Files          45       48       +3     
  Lines       11097    11134      +37     
  Branches     2062     2071       +9     
==========================================
+ Hits        10757    10764       +7     
- Misses        292      317      +25     
- Partials       48       53       +5

Impacted Files	Coverage Δ
relate/utils.py	95.69% <ø> (ø)	:arrow_up:
cas_config/__init__.py	0% <0%> (ø)
cas_config/receivers.py	0% <0%> (ø)
cas_config/apps.py	0% <0%> (ø)
course/auth.py	84.08% <0%> (-0.41%)	:arrow_down:
relate/settings.py	72.09% <0%> (-5.41%)	:arrow_down:
relate/urls.py	81.48% <0%> (-18.52%)	:arrow_down:
course/page/text.py	100% <0%> (ø)	:arrow_up:
... and 1 more

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[Mihara]

There, took care of all the linting issues.

[Mihara]

Well, since I no longer work there, there's no point keeping this around.