Add a way to mention people in topics

[hakubo]

So they could find all topics that they were mentioned in

This helps if agenda is made for large group of people and you want to find out if you should go to a meeting or you want to find out if someone needs your help/input

[ceejbot]

This is a good suggestion!

[ceejbot]

How-to: use @email to mention somebody. The mention should automatically add the topic to the mention-ee's list of relevant topics. If we implement notifications ever, it should trigger a notification.