baseUri doesn't work if it is relative

[wibblymat]

The example in the docs is path/to/directory, and the default when document.currentScript is unavailable is ./. But both of these would actually fail to work.

platinum-sw-cache.html#L143 calls the URL constructor with the baseUri as the second parameter but this MUST be an absolute URL.

> new URL('bootstrap/sw-toolbox-setup.js', './')
Uncaught TypeError: Failed to construct 'URL': Invalid URL(…)

We need to do an additional step somewhere to convert any relative baseUri into an absolute one.