previously we'd skip the template page entirely if someone didnt have
manage templates/api keys permission. however, if the template is
deleted you'd then go through the flow entering placeholders and stuff
before it would then crash when trying to send.
instead, just bounce the user to the template page. It has the content
and says when the template was deleted.
Previously this was duplicated between the "two_factor" and the
"webauthn" views, and required more test setup. This DRYs up the
check and tests it once, using mocks to simplify the view tests.
As part of DRYing up the check into a util module, I've also moved
the "is_less_than_days_ago" function it uses.
This provides more room for expansion, so we don't get another
massive file to scroll through. We do also have some top-level
files, such as "formatters.py", which we could consider moving
under utils/ in future.
