[LIFT] Support export/import of entry flags

[imnasnainaec]

Resolves #3264
Depends sillsdev/libpalaso#1414

Also, drops Backend LIFT support of entry Plural, which is otherwise unsupported.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 73.18%. Comparing base (d09ce63) to head (17a2970).
Report is 27 commits behind head on master.

Files with missing lines	Patch %	Lines
Backend/Services/LiftService.cs	52.63%	15 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3677      +/-   ##
==========================================
- Coverage   73.32%   73.18%   -0.15%     
==========================================
  Files         285      285              
  Lines       10656    10673      +17     
  Branches     1327     1332       +5     
==========================================
- Hits         7814     7811       -3     
- Misses       2450     2467      +17     
- Partials      392      395       +3     

Flag	Coverage Δ
backend	82.46% <60.00%> (-0.37%)	⬇️
frontend	65.91% <ø> (ø)

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[jasonleenaylor]

Reviewed 7 of 7 files at r2, 1 of 1 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)