Allow Form->forTemplate() URL access (fixes #788)

Need to specifically whitelist URL-accessible actions now.
Used in "Insert Link" form in HtmlEditorField.
Regression from 1edf45fbedd1431f7b0105403b628deda2b61bdc

It would be nicer to make this an `index()` method,
and allow access via `mycontroller/MyForm` rather than `mycontroller/myForm/forTemplate`,
but that doesn't work because actions fall back to `Form->httpSubmission()` at the moment.