Backend Refactor

[johnthagen]

Closes #882

Follow on to #892

Cleanup:

• Force all Backend.Tests compile warnings into errors
• Fix WordRepository.GetWord and WordService.Delete type signatures to properly return null on failure
  • Includes fixing the type signatures and implementations of Mocks to correctly handle missing entries
• Fix implementation and signature of UserEditRepository.GetUserEdit & UserEditApiServices to properly handle missing IDs
• Fix type signature and implementation of UserController.Get
• Fix UserRoleApiServices.GetUserRole to properly handle missing IDs
• Update UserApiServices.GetUserAvatar to properly handle missing user IDs
• Remove redundant parentheses
• Fix typos
• Remove unused variables
• Remove null checks proven by the compiler to not be possible
• Make all custom Exceptions Serializable
• Use Assert.IsInstanceOf<>() to simplify tests

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This change is 

[codecov-io]

Codecov Report

Merging #895 (88c6f24) into master (aa6cfa2) will decrease coverage by 0.15%.
The diff coverage is 29.54%.

@@            Coverage Diff             @@
##           master     #895      +/-   ##
==========================================
- Coverage   50.91%   50.76%   -0.16%     
==========================================
  Files         241      241              
  Lines        6597     6631      +34     
  Branches      423      423              
==========================================
+ Hits         3359     3366       +7     
- Misses       2922     2949      +27     
  Partials      316      316              

Flag	Coverage Δ
backend	55.89% <29.54%> (-0.37%)	⬇️
frontend	45.57% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
Backend.Tests/Helper/Sanitization.cs	100.00% <ø> (ø)
Backend/Controllers/WordController.cs	53.39% <ø> (+3.39%)	⬆️
Backend/Helper/FileStorage.cs	89.04% <ø> (ø)
Backend/Services/LiftApiServices.cs	93.34% <ø> (ø)
Backend/Services/PermissionService.cs	0.00% <0.00%> (ø)
Backend/Services/UserApiServices.cs	0.00% <0.00%> (ø)
Backend/Services/UserEditRepository.cs	0.00% <0.00%> (ø)
Backend/Services/UserRoleApiServices.cs	0.00% <0.00%> (ø)
Backend/Services/WordRepository.cs	20.63% <0.00%> (-1.40%)	⬇️
Backend/Startup.cs	0.00% <ø> (ø)
... and 11 more

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[imnasnainaec]

Reviewed 3 of 3 files at r1, 36 of 36 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved