Make Service Worker the default if the app is being accessed as a PWA

[Jaifroid]

This would be a step towards #196. It would be a very short PR (probably one-liner). It makes no sense that accessing the app as a PWA -- which requires Service Worker by definition -- opens the app in JQuery mode. So, the idea here is that if the app is being accessed from http(s): and Service Worker is available, it should open in SW mode by default (the user can still change back to JQuery).

This would not yet affect our extensions, which would open in JQuery mode by default.

This would also fix #801 implicitly. I'd recommend this being part of release 3.3.