Rename `PodURI` to `PodBaseURL` to better describe what kind of URL this
returns. Also renamed `*URI` to `*URL to be consistent with the `.URL`
field of the Single-User User-Agent.
The `MultiUserAgent.PodBaseURL()` method returned the wrong base URL
when yarnd serves from a subdirectory. The `../..` was too much. Thanks
to the new tests, this got discovered.
Refactor duplicated code into a superclass. The only drawback is the
field initialization, which is quite ugly now. :-(