Arbitrary mount paths

[trieloff]

Helix Pages currently assumes that it always serves from the root directory, which is not the case of larger sites.

For instance if adobe.com wants to serve adobe.com/products/helix from the adobe/helixsite repo, complicated path manipulations have to be made in the outer CDN.

One step to make it easier would be to support URLs like main--helixsite--adobe--products-helix/products/helix, i.e. treat everything after the owner as mandatory path fragments that will specify the root path to serve the site from.

[trieloff]

We'd still need to address the assumption that we have access to the /hlx_* namespace by replacing it with a *.hlx pseudo-root.