WIP: Add "skeletoning"/anonymizing of user applications

[wes-otf]

Would close #3441. Adds a new ApplicationSubmissionSkeleton model that only contains minimal, non-PII datapoints. Also adds UI features for staff to select whether a user's application(s) should be deleted or converted to skeletoned/anonymized application.

The todos:

• Implement ApplicationSubmissionSkeleton model & creation methods
• Handle manual individual submission deletions
• Handle manual bulk submission deletions/anonymization
• Handle submissions with individual user deletions
• Handle submissions with bulk user deletions
• Include anonymized submissions in the results dashboard
• Add unit tests for new functionality

Test Steps

• …