Add new query for MP activity report

This gives a rolling count of how many players have played how many games, which is useful for seeing what player retention for MP is.

utils/mp-server/query-scripts/28-count-of-players-by-count-of-games.sql

@@ -0,0 +1,19 @@
+select COUNT_OF_GAMES, count(*) as COUNT_OF_USERS
+from
+(
+  select USER_ID, count(*) as COUNT_OF_GAMES
+  from
+  (
+    select player.USER_ID
+    from wesnothd_game_player_info player, wesnothd_game_info game
+    where player.USER_ID != -1
+      and player.INSTANCE_UUID = game.INSTANCE_UUID
+      and player.GAME_ID = game.GAME_ID
+      and game.END_TIME is not NULL
+      and TIMESTAMPDIFF(MINUTE, game.START_TIME, game.END_TIME) > 5
+    group by player.INSTANCE_UUID, player.GAME_ID
+  ) i1
+  group by USER_ID
+) i2
+group by COUNT_OF_GAMES
+order by COUNT_OF_GAMES