[Nateowami]

This removes #nullable disable warnings where it is possible to do so without breaking things, or where only very minor changes are needed.

It introduces non-null assertions and marks some things as optional that were previously not marked as optional. However, in general this is merely making the code more honest about what is optional and when it is doing something the compiler can't prove is safe, NOT making something unsafe that was safe before.

The main benefit isn't to the code that is updated here, but the fact that lots of future code will be more strictly checked for type safety.

This change is 

[codecov]

Codecov Report

❌ Patch coverage is 65.90909% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.31%. Comparing base (c429a36) to head (3d91a8c).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ure/Services/MachineServiceCollectionExtensions.cs	0.00%	5 Missing ⚠️
src/SIL.XForge.Scripture/Startup.cs	28.57%	5 Missing ⚠️
...XForge.Scripture/Controllers/ParatextController.cs	63.63%	3 Missing and 1 partial ⚠️
...IL.XForge.Scripture/Services/DotNetCoreRegistry.cs	20.00%	4 Missing ⚠️
src/SIL.XForge/Models/InterfaceLanguage.cs	0.00%	4 Missing ⚠️
...ge/Services/JsonRpcApplicationBuilderExtensions.cs	0.00%	3 Missing ⚠️
src/SIL.XForge.Scripture/Migrator.cs	0.00%	2 Missing ⚠️
...IL.XForge.Scripture/Models/EditorTabPersistData.cs	0.00%	2 Missing ⚠️
...c/SIL.XForge.Scripture/Models/OnboardingRequest.cs	33.33%	2 Missing ⚠️
...L.XForge.Scripture/Services/SFScrTextCollection.cs	0.00%	2 Missing ⚠️
... and 12 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4252      +/-   ##
==========================================
+ Coverage   81.30%   81.31%   +0.01%     
==========================================
  Files         671      671              
  Lines       43020    43086      +66     
  Branches     7138     7119      -19     
==========================================
+ Hits        34976    35035      +59     
- Misses       6877     6899      +22     
+ Partials     1167     1152      -15     

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