Link existing link if one exists

[VMelnalksnis]

Fixes #356.

Changes proposed in this pull request:

• Search for an existing link with the same value before adding it to a transaction
• Return 409 if a link already exists with the same value

[codecov]

Codecov Report

Merging #357 (ced102b) into master (3298f09) will increase coverage by 0.14%.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##           master     #357      +/-   ##
==========================================
+ Coverage   67.32%   67.47%   +0.14%     
==========================================
  Files         247      247              
  Lines        6923     6942      +19     
  Branches      489      494       +5     
==========================================
+ Hits         4661     4684      +23     
+ Misses       2100     2090      -10     
- Partials      162      168       +6     

Impacted Files	Coverage Δ
...omeshade.Interfaces.WebApi/V1_0/LinksController.cs	72.72% <33.33%> (-20.38%)	⬇️
....Core/Transactions/Links/LinkUpsertionViewModel.cs	72.00% <100.00%> (+48.19%)	⬆️
...ade.TestingHelpers/Data/Fakers/NamedEntityFaker.cs	57.14% <0.00%> (-14.29%)	⬇️
...onia.Core/DesignTime/DesignTimeGnomeshadeClient.cs	74.59% <0.00%> (+1.95%)	⬆️

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