Handle old serviceworkers

[atjn]

If an old serviceworker was on the site before, it needs to still be there for the browser to update it and thereby discover that the EWAB serviceworker now exists instead. This can be achieved by simply placing an empty file on the exact path that the old serviceworker was on.
It could be a built-in feature of EWAB to keep track of old serviceworkers and update them with empty files.

EWAB should also have some way of updating its own serviceworkers in case there is a name-change (currently a thing when the user updates the EWAB alias). It could be the above-mentioned feature for simplicity, or something more complicated, like ensuring that the serviceworker update file is always redownloaded.