Refactor importing, move bank reference to external

[VMelnalksnis]

Fixes #241

Changes proposed in this pull request:

• Move bank reference values to external reference
• Populate both bank and external reference when importing

[codecov]

Codecov Report

Merging #738 (c6ffeeb) into master (e4bc220) will increase coverage by 1.48%.
The diff coverage is 85.64%.

@@            Coverage Diff             @@
##           master     #738      +/-   ##
==========================================
+ Coverage   73.46%   74.95%   +1.48%     
==========================================
  Files         221      225       +4     
  Lines        6347     6281      -66     
  Branches      416      380      -36     
==========================================
+ Hits         4663     4708      +45     
+ Misses       1510     1441      -69     
+ Partials      174      132      -42     

Impacted Files	Coverage Δ
...ce/Gnomeshade.Data.Sqlite/Dapper/DecimalHandler.cs	50.00% <50.00%> (ø)
...eshade.WebApi/V1/Controllers/NordigenController.cs	82.89% <66.07%> (+12.68%)	⬆️
...de.WebApi/V1/Importing/TransactionImportService.cs	86.26% <86.26%> (ø)
...eshade.WebApi/V1/Controllers/Iso20022Controller.cs	91.78% <94.87%> (+15.84%)	⬆️
...meshade.Data.Sqlite/ServiceCollectionExtensions.cs	100.00% <100.00%> (ø)
...Gnomeshade.Data/Repositories/TransferRepository.cs	94.11% <100.00%> (+1.26%)	⬆️
...ource/Gnomeshade.WebApi.Client/GnomeshadeClient.cs	80.43% <100.00%> (+0.14%)	⬆️
...ource/Gnomeshade.WebApi.Client/SuccessfulImport.cs	100.00% <100.00%> (ø)
.../Importing/ImportingServiceCollectionExtensions.cs	100.00% <100.00%> (ø)
...shade.WebApi/V1/Importing/Iso20022ImportService.cs	100.00% <100.00%> (ø)
... and 3 more

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