As Mail::Sender is deprecated, libmail-sender-perl is
no longer available on e.g. debian stretch. Updating to
use Email::Sender and libemail-sender-perl works well.
- change the secret/secrets code so that it will work in both old and new
Mojolicious versions.
- Add information that perl modules can be installed from distribution
package repositories, and not neccessarily from cpan
- Add more description to information on config file
- Revert default config to previous state
- Add tests (to be run by explain.pl test) that check for required perl
modules
