Add flag value to database

Unfortunately required, as loadBeatmaps() refreshes the decoded beatmap
with the database-stored values, which can end up overwriting the
decoded ones.

osu.Game/Beatmaps/BeatmapInfo.cs

@@ -90,8 +90,6 @@ namespace osu.Game.Beatmaps
 
         public bool LetterboxInBreaks { get; set; }
         public bool WidescreenStoryboard { get; set; }
-
-        [NotMapped]
         public bool EpilepsyWarning { get; set; }
 
         // Editor