Add ability to link a refund to the original transaction

[VMelnalksnis]

Fixes #179

Changes proposed in this pull request:

• Add RefundedBy to transactions
• Add UI to link the refund transaction to the original transaction

[codecov]

Codecov Report

Merging #721 (d7e0921) into master (e2fdc64) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #721      +/-   ##
==========================================
+ Coverage   72.26%   72.27%   +0.01%     
==========================================
  Files         219      219              
  Lines        6305     6309       +4     
  Branches      412      412              
==========================================
+ Hits         4556     4560       +4     
  Misses       1580     1580              
  Partials      169      169              

Impacted Files	Coverage Δ
...urce/Gnomeshade.Data/Entities/TransactionEntity.cs	100.00% <100.00%> (ø)
...omeshade.WebApi.Models/Transactions/Transaction.cs	100.00% <100.00%> (ø)
....WebApi.Models/Transactions/TransactionCreation.cs	100.00% <100.00%> (ø)

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