Add getAvailablePathRelativeToRoot() method

[adriaanzon]

Adds a method similar to getAvailablePath() that calls getPathRelativeToRoot() instead of getPath().

This allows code like the following, in cases where you use the local filesystem driver in your development environment and the s3 driver in production, as getPath() returns the absolute path which you cannot pass to Storage::get().

Storage::disk($media->disk)->get(
    $media->getAvailablePathRelativeToRoot(['conversion'])
)

[freekmurze]

Some of the tests are failing, could you have a look?

[adriaanzon]

This is an issue in Testbench: orchestral/testbench-core#368