Fix cg.intermissionStarted only being enabled at first intermission

Game sets CS_INTERMISSION to 1 at intermission, but does not clear it at
map_restart so it's only sent the first time. CG_MapRestart manually clears
cg.intermissionStarted (which is set to value of CS_INTERMISSION when it's
modified). So subsequent intermissions do not have cg.intermissionStarted
enabled.

Now CS_INTERMISSION is cleared and sent each time intermission is started
and cg.intermissionStarted is enabled each time.

This makes subsequent intermissions not play sounds in CG_CheckLocalSounds
during the 1 second between intermission starting and switching to scoreboard
(PM_INTERMISSION) and makes Team Arena voice chats not play.

code/game/g_main.c

@@ -506,6 +506,7 @@ void G_InitGame( int levelTime, int randomSeed, int restart ) {
 
 	G_RemapTeamShaders();
 
+	trap_SetConfigstring( CS_INTERMISSION, "" );
 }