Add past behaviour clause to CoC

[RichardLitt]

I've had at least one comment that it is important to think about past behaviour for moderators, particularly in cases where there is legal precedence for a person being unsafe in a given community based on past experiences in similar communities.

Outreachy has this clause:

If a concern about a past incident in any space is brought up about a current Outreachy community member, Outreachy organizers may take it into account when deciding whether the incident should have any consequences to the community member's participation in the Outreachy community.

I think we should add something similar to the CoC at the next review, in late June.