Expose alert.mp3 in the installed folder

[neilenns]

Low-priority feature request, but it'd be great if alert.mp3 was exposed as a standalone file in the install instead of being packed with webpack so users can replace it with a sound of their own.

CRC drops all the audio files in a standalone folder which has been super handy to replace with different sounds that are easier to pick up in the cacophony of sounds that happen during ATC sessions.

[pierr3]

Will potentially add