Django Strategy: support for X-Forwarded-* headers

[e-kolpakov]

Description: This PR adds support for getting host and port from X-Forwarded-* headers, if they are available.

Related: edx/edx-solutions-edx-platform#530, https://github.com/edx/edx-platform/pull/9848

Motivation: If an application using PSA is running behind the reverse proxy on a non-standard port, PSA (python-saml, to be precise) fails checking the SAML assertion response with message "Authentication failed: SAML login failed: ['invalid_response'] The response was received at http://host:port instead of http://host".

Common way of passing original host and port to proxied app is using X-Forwarded-* headers. Some webservers (i.e. gunicorn) already respect those headers and pass them to django as "real" host and port (i.e. request.get_host() and request.META['SERVER_PORT'] actually return X-Forwarded-Host and X-Forwarded-Port` values). Other webservers (i.e. django dev server, nginx(?)) does not do so.

This PR adds a web-server-agnostic way to use PSA behind the reverse proxy.

Author concerns:

1. This might not be an optimal way to do it. Possible alternatives:
   1.1. It is already working fine; clients should either use web-servers that respect X-Forwarded-* headers, or solve this issue in some other, non-PSA-related way.
   1.2. Clients should provide custom strategy with request-host and request_port overridden (i.e. follow the same approach as in https://github.com/edx/edx-platform/pull/9848)
2. There seems to be serious complications to testing frameworks-specific code - there're no tests for framework-specific strategies so far, and adding a test for django strategy required adding Django, specifying settings and so on - and eventually blowed up entire test suite. Hence tests are not included so far, but I would be glad to add them, if some architectural guidance is given.

[e-kolpakov]

@omab I'm not sure who should I talk to to get this merged, so starting with you. The test failure is something that happened with all the other PRs opened at that time, plus it only happens to pypy version of it (is there a problem with test infrastructure). So, what would be the next steps to get this merged (or at least discussed)?

[omarkhan]

@e-kolpakov don't you want to return the port here rather than the host?

[e-kolpakov]

@omarkhan good catch

[e-kolpakov]

Closing in favor of #841