Make contributions restful and authorized

[solebared]

Why

Completes authorization changes to ContributionsController in preparation for re-opening registration.

What

1. Removes unused combined_form action and view
2. Extracts thank_you action into its own controller
3. Change triage action to edit
4. Change triage_update action to update
5. Flesh out ContributionPolicy
6. Authorize all actions in ContributionController (except index)

Next Steps

?

Outstanding Questions, Concerns and Other Notes

• We should put a policy_scope around the index action but i ran out of time to do that.

Pre-Merge Checklist

• All new features have been described in the pull request
• Security & accessibility have been considered
• High quality tests have been added, or an explanation has been given why the features cannot be tested
• New features have been documented, and the code is understandable and well commented
• Entry added to CHANGELOG.md if appropriate
• All outstanding questions and concerns have been resolved
• Any next steps that seem like good ideas have been created as issues for future discussion & implementation