Instruct user to move app to /Applications if launched from somewhere else

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Instruct user to move app to /Applications if launched from somewhere else.

Is your feature request related to a problem? Please describe.

Sparkle updates can fail otherwise (#70).

What would you like to see? How would you like it to work?

Something like this, which is common for macOS apps distributed outside the Mac App Store: https://github.com/potionfactory/LetsMove/. I'm not sure if this is the best implementation, it's just the first one I found, so do some more research on this before implementing.