Improve performance of admin/conference/*/registrations

• Add some caching
• Reduce query count, including N+1s on questions & roles

Checklist

• I have read the Contribution & Best practices Guide.
• My branch is up-to-date with the upstream master branch.
• The tests pass locally with my changes.

Short description of what this resolves/which issues does this fix?:

• Registrations page was so slow it wouldn't load anymore (Total render time: 123 seconds)

Changes proposed in this pull request:

• Add some caching
• Reduce query count, including N+1s on questions & roles

Total render time: 6 seconds. Still slow, but, you know, 1/20 of the time before ;-)