Keep assignee on onboarding requests even after resolving

[Nateowami]

Previously resolving an onboarding request cleared the assignee. It was requested that the assignee stay on the request, so it's possible to look back at requests and see who handled them.

This necessitated two changes:

1. Stop clearing the assignee when the request is resolved
2. Update the status calculation to not assume that a request that has an assignee is in progress.

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This change is 

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.82%. Comparing base (840cc41) to head (14ff8ff).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...c/SIL.XForge.Scripture/Models/OnboardingRequest.cs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3691   +/-   ##
=======================================
  Coverage   81.82%   81.82%           
=======================================
  Files         619      619           
  Lines       38619    38618    -1     
  Branches     6317     6317           
=======================================
  Hits        31601    31601           
+ Misses       6044     6043    -1     
  Partials      974      974           

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