Postorius should present current template to edit when it is still default

Specifically footer, but probably lots suffer from a lack of editing template when it's default.

See Mark's message https://mail.python.org/archives/list/python-ideas@python.org/message/HBBZF6DIPDISNPXPLGZ5XBUMJCBFHPJY/.

Note that since Mark didn't say "I'll fix that right up", I'm guessing this requires cooperation from the core, which probably doesn't "know" what the template is in the REST interface until it's been customized through Postorius.