Ensure we only save the "cleaned" model to the DB

* Extract the model cleaning logic to a method for re-use
* Call cleaning method from ConvertEditorToDb  (we were only calling on ConvertDbToString (cache))
* Update ConvertDbToEditor to route through new deserialization helper, so that it gets the "virtual" properties it needs that were lost in the cleaning
Persist without formatting/whitespace
Show "remove" button when MaxFieldsets = 1 and Multiple Fieldset Types

... so the user can switch types (remove/add) when only a single row.

We check for Multiple Fieldset Types as well, because we don't want to show a "remove" button when there's only one type allowed (presumably).
Use a count on the add button rather than checking flag