theme: allow to be served by file:// protocol

[McShelby]

Serving the theme by file:// protocol requires uglyURLs=true and relativeURLs=true.

Sadly for the home page, this does not generate links to <mypath>/index.html but only <mypath>/. This makes the homepage inaccessible.

Furthermore, search can not load the required <mypath>/index.json.

There also may be further issues when using markdown links or the relref shortcode. This needs further examination.

[McShelby]

Markdown links are not touched yet but may be part of a new ticket if this proves to be an issue.