[MM-17422] Added code to update, delete last bot_icon time in model.bot. (#12229)

Display LHS bot Icon in web app. As part of mentioned task, Added LastIconUpdate variable in model.bot to store last update time of icon. Also added code to update/delete value of the mentioned variable when setting/deleting bot icon.

store/sqlstore/user_store.go

@@ -60,7 +60,7 @@ func NewSqlUserStore(sqlStore SqlStore, metrics einterfaces.MetricsInterface) st
 	}
 
 	us.usersQuery = us.getQueryBuilder().
-		Select("u.*", "b.UserId IS NOT NULL AS IsBot", "COALESCE(b.Description, '') AS BotDescription").
+		Select("u.*", "b.UserId IS NOT NULL AS IsBot", "COALESCE(b.Description, '') AS BotDescription", "COALESCE(b.LastIconUpdate, 0) AS BotLastIconUpdate").
 		From("Users u").
 		LeftJoin("Bots b ON ( b.UserId = u.Id )")