Add ability to use offline (potentially through local storage)

[dcallus]

Currently (at least on iphone) when you save the app to your mobile desktop and attempt to use offline, the app works, except for the sounds.

I think the solution would be to store the sounds in local storage which there is information about on stack overflow. But perhaps there are other solutions.

sounds files are inside react-pro-metronome library in the source. It's okay to modify this library, it already is.

[dcallus]

Note: before this is done, we might need change the app to not have so many different media types. Or perhaps it will work by just saving mp3 or wav for example.