SF-3664 Fix projects creating duplicate Paratext User entries

[RaymondLuong3]

This issue happens when a user began as a Username-Password user that was later linked to a Paratext identity. When a user joins a project, if the user happens to have an alternate username on the user doc different than in Auth0, then a duplicate paratext user entry will be added to the ParatextUsers collection on the project. Some projects on QA, and on 1 project on live experienced this. The only issue this poses is that multiple users may appear in the users list.
This change ensures that we use the paratext username as the key to determine if a new entry should be inserted into the ParatextUsers collection on a project.

Cleaning up the QA and live projects will be a separate task.

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This change is 

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.52%. Comparing base (2922a00) to head (936c4e1).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
.../SIL.XForge.Scripture/Services/SFProjectService.cs	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3634      +/-   ##
==========================================
- Coverage   83.52%   83.52%   -0.01%     
==========================================
  Files         610      610              
  Lines       37504    37509       +5     
  Branches     6170     6171       +1     
==========================================
+ Hits        31327    31329       +2     
- Misses       5224     5226       +2     
- Partials      953      954       +1     

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[pmachapman]

@pmachapman reviewed 2 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @RaymondLuong3).