Clean up use of relative URLs. Fixes #844 and #845 (#878)

* Handle links to empty-string href, resolved against base href

Needed to change the URLs used in E2E tests to be able to cover this (i.e., removed the /RouterTest prefixes so the default relative URL became an empty string)

* Change links in StandaloneApp sample to be relative

* Standardize on base URIs having trailing slash everywhere

Hence also change terminology from "base URI prefix" to simply "base URI"

* Handle link highlighting when visiting base-href-but-without-trailing-slash

* Removing leading slashes from base-relative URLs in templates