Look for installed Parity in more locations

[axelchalon]

Currently, Parity UI only checks if Parity exists in Parity UI's AppData folder (i.e. only checks if it had already downloaded Parity). If it doesn't exist, it downloads Parity. Would be better to check if Parity is installed in the system in some standard locations so that we don't download a second instance of Parity.

This is already implemented in Fether in the @parity/electron package:
https://github.com/paritytech/fether/blob/master/packages/parity-electron/src/getParityPath.ts

@parity/electron (currently in the fether repo) will soon be moved to a shared repo paritytech/js-libs, so we can just wait until it's moved there and then use @parity/electron inside Parity UI.