V12/block grid independence

[KevinJump]

Investigating: #538

Change how we serialize information back info umbraco for block lists.

note: The current way "works" but deploy doesn't like it (do we cares what deploy likes 😉 )

current we serialize the Json in and out as Json all the way down a blocklist. This makes it look nice, and it's easier to track changes, and source code commits, and Umbraco works with this being stuck back into the db.

but technically, block list child values are escaped Json in the DB. this is probibly more efficient 😛 - but it makes for hard to decipher text.

This update now puts the values into the DB in their ugly form.
but it forces the uSync file format to be nice Json, so we keep our nice diff/source control.

The main thing here is separating Blocklist a bit more from the way Nested Content does things, I was going to completely separate it, but in reality, two new abstract methods GetImportProperty and GetExportProperty are enough as they give us the granular control over what we do with a value once it's been mapped.

[KevinJump]

As the changes are in theory only to how block lists serializer (we have moved the based import and export into methods so we can override them).

that should mean no changes to other stuff, but i have been burnt before with this - so need to do some checks make sure

• all then properties still get set,
• they still render on the front end
• the back end still loads them all up