Previously, the path returned by `LocationService.path()` preserved leading
slashes, which resulted in requests with consequtive slashes in the URL. Such
requests would fail (with a 404) on staging.
This commit fixes it, by removing leading slashes from the path. It also
refactors `LocationService` a bit, converting path to an observable,
`currentPath` (similar to `currentUrl`), and applies certain clean-ups (e.g.
stripping slashes, query, hash) in one place, which simplifies consumption.
Closes#16230
