Added party alliances

Adds the following commands:
`/party alliance`
`/party alliance invite <target>`
`/party alliance accept`

Allies will share party chat and will not be able to harm eachother.
Item and XP sharing is limitied to a single party. Allowing two parties
to team up without sharing resources and XP.

Closes #1009

src/main/resources/config.yml

@@ -386,6 +386,7 @@ Commands:
     partychat:
         Chat_Prefix_Format: '[[GREEN]]([[WHITE]]{0}[[GREEN]])'
         Use_Display_Names: true
+        Chat_Prefix_Format_Ally: '[[GREEN]](A)[[RESET]]'
     adminchat:
         Chat_Prefix_Format: '[[AQUA]][[[WHITE]]{0}[[AQUA]]]'
         Use_Display_Names: true