Allow custom templates for broadcasting (mail) facilities

[DonDebonair]

Currently, only the standard template is available for mails sent by Athena. It should be possible to provide custom (Jinja2) templates and pass the name of the desired template over the command line. It should also be possible to configure a default template.

Athena should do the following a command is invoked that uses a template:

• If the name of a template is passed along, try to find that template in $CONFIGDIR/templates/, and if it does not exist there, try to find it in the Athena template dir.
• If no template name is passed, use the default as specified in the configuration file, or if that has not been configured, use the Athena default.

As an optional feature, we could allow passing a full path to a template.