Refactor superuser API endpoint to migrate Shib user to builtin/local account

[pdurbin]

I had coder's remorse over the quality of work I put into the superuser API endpoint to migrate Shib user to builtin/local account in #2915 and confessed to @scolapasta at #3025 (comment) . We agreed that the code should be refactored, which I did in this pull request. Nothing changes in terms of how the API works. Docs at http://guides.dataverse.org/en/2939-shib/installation/shibboleth.html#converting-shibboleth-users-to-local are still accurate. The code was cleaned up and centralized, which means it will be easier to build a GUI around this feature in the future. I'm reopening #2915 since it hasn't shipped yet.

1. Related Issues

• Shibboleth: Add superuser API endpoint to migrate Shib user to builtin/local account #2915 Shibboleth: Add superuser API endpoint to migrate Shib user to builtin/local account

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2. Pull Request Checklist

• Functionality completed as described in FRD
• Dependencies, risks, assumptions in FRD addressed
• Unit tests completed
• Deployment requirements identified (e.g., SQL scripts, indexing)
• Documentation completed
• All code checkins completed

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3. Review Checklist

After the pull request has been submitted, fill out this section.

• Code review completed or waived
• Testing requirements completed
• Usability testing completed or waived
• Support testing completed or waived
• Merged with develop branch and resolved conflicts

[coveralls]

Coverage increased (+0.0006%) to 5.145% when pulling cd2b9cc on 2915-shib2builtin into 7fe7040 on develop.

[kcondon]

OK to merge