Bounce tracking mitigations for dual-use sites (URL-level BTM)

A place for discussing [the explainer](https://github.com/privacycg/nav-tracking-mitigations/blob/main/explainers/bounce-tracking-mitigations-for-dual-use-sites.md) for extending bounce tracking mitigations to dual-use sites.

Some known open questions have dedicated issues for discussing them (see "Sub-issues" section below). Discussions of those topics should take place on those issues.