2.6: Startup is slow due to GameNotesMigration.extractGameNotes().

[asvitkine]

I profiled TripleA start up and I noticed that the main thing responsible for a slow startup and taking a while to show the initial game screen is GameNotesMigrator.extractGameNotes(). It was taking ~5 seconds.

I don't think we should be doing this for all maps synchronously on startup. We should either do it in the background, not blocking startup, or do it on demand when needed by a map.