urls: allow canonical_url to contain more than just a hostname

Although the .ini file gives the example:

canonical_url = https://kallithea.example.com/repos

it does not actually work. The '/repos' part is stripped off by the
canonical_url method.

The 'host' entry in the arguments passed to routes.url does not strictly
need to be a pure hostname. At least, the implementation does no validation
of this fact, it is concatenated verbatim between the protocol and the rest
of the URL.

As mapping Kallithea to a subpath of a base hostname is a valid
implementation, the canonical_url feature should allow it.