feature: default to including site sign-in URL in all PC member emails

[kaytwo]

I receive a lot of hotCRP emails. For whatever reason, many of them that include a specific call to action related to doing my hotCRP work do not include the log in url for the specific conference that they're a part of. As one could imagine, this drives me nuts and is a big speedbump to overcoming the activation energy needed to actually do my Very Important Volunteer Service to the Academic Community™️.

Adding this URL automatically to outgoing PC emails may well be a feature that already exists, but for whatever reason PC chairs don't seem to use it ever.

I would LOVE it if hotCRP could make it easier to default to having all PC emails (or all emails) include a "signature" block including a log in link for the specific conference. If it just defaulted to "{Conference name} {conference year} login: {baseurl}" and is a default that must be specifically opted out of would make a big difference and shouldn't leak any sensitive information if it gets added to emails unintentinally.

@kohler I will write the PR if this is a feature you'd be interested in.

[kohler]

HI @kaytwo! This is a nice idea. It should be pretty simple to do; just edit the existing templates in etc/mailtemplates.json.