Stream all file downloads

[johnthagen]

• This reduces peak/persistent memory usage of downloading a 116MB Lift export by ~116MB.

Closes #811

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This change is 

[johnthagen]

Avatar display issues are unrelated to this PR: #879

[codecov-io]

Codecov Report

Merging #878 (e80d027) into master (28f2dfa) will not change coverage.
The diff coverage is 68.00%.

@@           Coverage Diff           @@
##           master     #878   +/-   ##
=======================================
  Coverage   51.01%   51.01%           
=======================================
  Files         241      241           
  Lines        6590     6590           
  Branches      418      418           
=======================================
  Hits         3362     3362           
  Misses       2918     2918           
  Partials      310      310           

Flag	Coverage Δ
backend	56.15% <68.00%> (ø)
frontend	45.84% <ø> (ø)

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

Impacted Files	Coverage Δ
Backend/Controllers/AvatarController.cs	57.89% <0.00%> (ø)
Backend/Controllers/FrontierController.cs	0.00% <0.00%> (ø)
Backend/Services/EmailService.cs	0.00% <ø> (ø)
Backend/Services/PermissionService.cs	0.00% <0.00%> (ø)
Backend/Controllers/AudioController.cs	52.63% <40.00%> (ø)
Backend/Controllers/ProjectController.cs	31.36% <55.55%> (ø)
Backend/Controllers/WordController.cs	50.00% <71.42%> (ø)
Backend/Controllers/LiftController.cs	52.72% <85.71%> (ø)
Backend/Controllers/UserController.cs	46.21% <100.00%> (ø)
Backend/Controllers/UserEditController.cs	63.24% <100.00%> (ø)
... and 1 more

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[johnthagen]

LGTM build failures unrelated and tracked in #883

[imnasnainaec]

Reviewed 12 of 14 files at r1, 1 of 1 files at r2, 1 of 1 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved

[jmgrady]

Reviewed 7 of 7 files at r4.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @johnthagen)

[imnasnainaec]

Reviewed 3 of 7 files at r4.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @johnthagen)