Switch to argparse in the previewer

Change-Id: Ia52f7dc8ef643bf423277d9cbc8bc0f1073aabdc
Error on templates which don't specify who they're to

This includes splitting out the group verification from the LDAP
connector (as the check is now supported via a class in libfritter).

Change-Id: Id941034a67184d35db8b0eb59776d91a100172ac
Avoid polling LDAP when previewing locally

Adding the requirement that users have an LDAP instance running in
order to preview a submission just complicates their setup without
providing much benefit. Instead, just check the existence of their
recipients against the config, and rely on that being correct.

Change-Id: I8c213a88a4cf47bd7f5f164524174220f4dac021