Update to CKAN 2.3+ and pysaml2 4.0 (and other enhancements)

We've been working on some updates and enhancements to this extension in our [fork](https://github.com/DataShades/ckanext-saml2). Top of the list of was updating the extension to run on a standard pysaml2 4.0 release with CKAN 2.3 and 2.4. pysaml2 has the same who.repoze dependency as these versions of CKAN so it fits together more easily. We've also worked on a number of enhancements for different working configurations.

We'd like to contribute this code, starting with the pysaml2 and CKAN updates (we should have done this earlier). Are you happy to receive PRs for these?

If so, the next question is how. We can construct a PR that includes all the upgrades changes only, which may take a little time, OR we could simply make a PR from our master which includes various new features also.

Review the proposed changes in the linked DataShades/ckanext-saml2 fork, focusing first on the CKAN 2.3/2.4 and pysaml2 4.0 updates. Clarify whether the contribution should contain only compatibility upgrades or also the fork's additional enhancements; done means an agreed PR scope and working contribution.

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