CON-1480 - Magic string as static value for 'Save as content node' workflow doesn't work.

PR for this is here - https://github.com/umbraco/Forms/pull/176

Test Notes

This came from a forum post originally - https://our.umbraco.org/forum/umbraco-forms/89999-using-magic-strings

• Create a new doctype (that has a few textstrings but mainly one for storing Form Record ID in)

• Create a form

• Create a workflow that uses the Save as Content Node workflow

• In the field mappings map form fields to document type

• For the Form Record ID in your doctype use the static value section and use the magic string of {record.UniqueId}

• Submit a form and verify that a content node is created as expected and the record guid/id is mapped across into the content node property as expected

The node is created upon form submit and all the fields where {record.UniqueId} has been used are now populated with the record guid. Nice job @warren.buckley :), merging.