Require the secret config to be configured

If it's not configured the instance will look like it is working but
various features will silently break (end to end encryption, setting
alternate email and probably more).

One issue is that changing the secret from empty to something will
break various other stuff (app token). I don't think there is a good way
to solve this issue other than breaking early instead of having to
handle a painful migration later on.

Signed-off-by: Carl Schwan <carl@carlschwan.eu>