Remove from friends list

Blocked by #22 The context menu from #10 should replace the "add to friends list" option with the "remove from friends list" option if the the player is already on the player's friends list.

Selecting the option will delete the relation between the two players from the perspective of the current player. (i.e. if the unfriended player has the current player on their friends list, then this relation will not be affected)