Player grouping

IT should be possible to group players together such that they can form a party.
The party should be persisted even if one or more of the players log off.
When in a party, the gold and xp that is earned by killing monsters is boosted by 15%. Loot chance should not be affected.

In the future, there will be raids with larger groups, and "general" fight rooms where the player will automatically be grouped with other players in the same room. That is out of scope for this issue.