Use OPEN_JSON for persisting tags

[linkdotnet]

This PR fixes #101

Open points:

• Does that work with MySQL?
• Migration?

Possible Migration for SqlServer:

ALTER TABLE [dbo].[BlogPosts]
ADD [Tags] [nvarchar](2048) NULL;
GO

UPDATE [dbo].[BlogPosts]
SET [Tags] = (
  SELECT '[' + STRING_AGG('"' + t.Content + '"', ',') + ']'
  FROM [dbo].[Tags] t
  WHERE t.BlogPostId = [dbo].[BlogPosts].[Id]
)

ALTER TABLE [dbo].[Tags]
DROP CONSTRAINT [FK_Tag_BlogPosts_BlogPostId]
GO

-- Drop the Tags table
DROP TABLE [dbo].[Tags]
GO