Override extension for replay files

[BElluu]

Closed #538

Waiting for changes from:

1. Set correct springsettings for engine #578
2. RecoilEngine #2975
   Changes in engine merged

[Hectate]

Per the electron-builder documentation, we need perMachine to be true to associate files on Windows. I thought we had that already, though, which is why that seems odd to me. I could be wrong.

I presume I can just rename a normal replay to have the new extension to test? That is to say; internally there is no difference in the replays, it's only the extension that has changed?

[BElluu]

Yea that's only extension name. You can change extension for any replay and test it.

[Hectate]

This seems good. The replays won't start using the new extension until the next engine release, I presume, but that's not going to stop the existing ones from working and it'll automatically pick up the new replays once they start being made. Thanks.