Odd handling of ID generation for LanguageToolCore rules on creation

[jonathonherbert]

For rules of type languageToolCore, we add a CHANGE_ME value to externalId when the rule is first written.

But it's perfectly possible to write some value to a rule when you first create it, while the rule is not a languageToolCore rule, and then switch the rule type to languageToolCore, exposing the UUID that was created on the initial write:

Yuk. We should use a separate property to store the languageToolCore ID used to refer to the LanguageTool rule, not externalId.