Unbind computed id from start_url

[marcoscaceres]

It would be good to unbind the computed id from the start_url, as the start_url could change. This leads to an issue where one set of users end up with one id, and a future set of users end up with a different id.

[benfrancis]

@marcoscaceres I agree this is a problem. However, considering the years-long analysis and discussion which arrived at the current solution, do you have an alternative in mind?

The biggest obstacle to changing it seems to be backwards compatibility with existing implementations.

A future Web App Manifest 2.0 specification could potentially make a breaking change. E.g. Make manifest URL the default identifier of a web app (my preference, requiring other significant simplifications), or make the id member mandatory (probably a more likely consensus).

See also: #668