Prevent duplicate friends appearing on the list

[michaeldyrynda]

Currently, if I add a friend to my conference list and they subsequently introduce themselves, that friend will end up on my list twice - once as a friend I plan to meet, and once as the introduced friend.

It should be trivial to modify the behaviour to use firstOrCreate when making an introduction. If the Friend is not new, add the introduction flag to that record.

[michaeldyrynda]

Closed by #51, but apparently, I don't know how to GitHub properly 😬