Default social media types

This is a bit more of a comprehensive change (albeit, for a single option).

In this PR, I propose that conferences may want to encourage attendees and speakers to give information about a certain number of their IM accounts. An example of this can be seen in the KDE conference system (an ofshoot of frab from some time ago) - in which the public_name has been repurposed to hold the IRC nick.

This method seems less intrusive to the codebase and easier to maintain.