Amend governance model to include rules on repeated terms for council members

We do not currently have any reference to whether a Council member can be re-elected at the end of their term.

This change makes the proposal that we should allow people to be re-elected for a maximum of two terms (six years) after which they must stand down for at least one term.

The idea behind this is that we don't get folks blocking others joining the Council by being re-elected over and over again. It gives a sufficient amount of time for someone to serve before taking a step back.

PR here: https://github.com/mautic/mautic-community-handbook/pull/418