[Web Install] Post-install updates

[dead-claudia]

After installing, a user might wish to update an application without immediately using it. UAs could also provide a centralized UI for checking and performing updates, like they already do for extensions.

A mobile user on the go will usually want to update while on Wi-Fi, but they might primarily use the app while on cellular, for instance.

This could be solved by just allowing UAs to manually fetch and update service workers without a window for installed web apps, to let them self-update.