[ProjectController] Update return types, Remove DeleteAllProjects

[imnasnainaec]

• Adds all missing ProducesResponseType attributes on controller functions
• Removes response object/body from functions with a Status304NotModified response, because it's invalid and we weren't using them
• Removes DeleteAllProjects because we'd never want to do that via a command from the frontend
• Adds tests

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 74.65%. Comparing base (79489c0) to head (0d6403b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
Backend/Controllers/ProjectController.cs	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3848      +/-   ##
==========================================
+ Coverage   66.05%   74.65%   +8.60%     
==========================================
  Files         236      287      +51     
  Lines        6057    10776    +4719     
  Branches      758     1347     +589     
==========================================
+ Hits         4001     8045    +4044     
- Misses       1809     2343     +534     
- Partials      247      388     +141     

Flag	Coverage Δ
backend	85.69% <75.00%> (?)
frontend	66.05% <ø> (ø)

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[jasonleenaylor]

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)