Update Word.Modified timestamp on merge or delete

[johnthagen]

Closes #960

• Update Word.Modified when
  • Words are merged
  • Words are deleted (e.g. removed from frontier)

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This change is 

[codecov-io]

Codecov Report

Merging #994 (5b3fe6b) into master (5f4387a) will decrease coverage by 0.05%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master     #994      +/-   ##
==========================================
- Coverage   49.79%   49.74%   -0.06%     
==========================================
  Files         236      236              
  Lines        6482     6489       +7     
  Branches      393      393              
==========================================
  Hits         3228     3228              
- Misses       2964     2971       +7     
  Partials      290      290              

Flag	Coverage Δ
backend	55.48% <44.44%> (-0.12%)	⬇️
frontend	43.53% <ø> (ø)

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Impacted Files	Coverage Δ
Backend/Services/WordRepository.cs	17.56% <0.00%> (-1.01%)	⬇️
Backend/Services/WordApiServices.cs	89.04% <100.00%> (+0.46%)	⬆️
Backend/Helper/Time.cs	50.00% <0.00%> (-50.00%)	⬇️

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[jasonleenaylor]

Yeah, I wish I could have worked out a good way to mock the database layer so that we could unit test our repositories, but mocking Mongo is too expensive and I'd prefer to avoid test databases.

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @johnthagen)