Add link to agent

This MR adds a link to the agent's interaction screen from diplomacy. I don't think there is any other obvious way linking them in master right now, short of searching for them in the locator. As I mentioned before, the longslave and slaveinteract sections have checks now for the agent and some restrictions in place unless they are recalled, so this should be safe to link now.

merged

mentioned in commit 2bef14ce

mentioned in merge request !6878 (merged)