Add a linked product to categories

[VMelnalksnis]

Fixes #720

Changes proposed in this pull request:

• Add ability to add shared database migrations to the data project instead of copying same SQL for all databases
• Create an optional product for each category, and automatically update it's name together with the category

[codecov]

Codecov Report

Merging #745 (6790ce3) into master (7aff5c4) will increase coverage by 0.20%.
The diff coverage is 94.82%.

@@            Coverage Diff             @@
##           master     #745      +/-   ##
==========================================
+ Coverage   77.04%   77.24%   +0.20%     
==========================================
  Files         225      226       +1     
  Lines        6298     6342      +44     
==========================================
+ Hits         4852     4899      +47     
+ Misses       1446     1443       -3     

Impacted Files	Coverage Δ
...rce/Gnomeshade.Data/Migrations/DatabaseMigrator.cs	86.66% <81.81%> (+12.47%)	⬆️
...hade.WebApi/V1/Controllers/CategoriesController.cs	91.04% <97.43%> (+7.71%)	⬆️
source/Gnomeshade.Data/Entities/CategoryEntity.cs	100.00% <100.00%> (ø)
...ade.Data/Migrations/MigrationScriptNameComparer.cs	100.00% <100.00%> (ø)
...urce/Gnomeshade.WebApi.Models/Products/Category.cs	100.00% <100.00%> (ø)
...meshade.WebApi.Models/Products/CategoryCreation.cs	100.00% <100.00%> (ø)

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