Auto download UnRAR.exe for Windows users (revisited) (#3354)

* Fix unescaped backslashes

* Reintegrate UnRAR.exe downloading for Windows

Note: unrar.exe will be hosted at sickrage.github.io

* Adjust change_unrar_tool more to be Windows drive, language and locale independent, and more reliable

* Adjust change_unrar_tool more

* Adjust change_unrar_tool to check all potential environment variables

* Forgot the unrar/unrar.exe

* Adjust usage 'Program Files' environment variables to account for
versions of Windows older than Vista and 32bit installations.

* Simplify path generation. Add inital test.

* Test further.

* Remove extra indent