Add support for story anchors

[privettoli]

Often there's a need for two people not to pair with each other because both of them anchored stories. Maybe I just missed and it's already available in PearTrix, so please correct me, but
Let's say we have:

1. Adam
2. Liz
3. Peter
4. Mr. Peterson
5. Andrew
6. Samuel

Adam and Mr. Peterson need to keep their stories and not pair with each other today, other team members can be randomized.