This fixes #2286

To do

Before we merge this and are stuck with this API until 0.5, let's consider:

• Do we want to merge functionality to 'enrich' or 'resolve' URLs first? E.g. check for Betty-specific schemes and rewrite URLs so something like betty://person/P1 becomes a link to the page of person P1 in the correct locale? (this could be helpful when loading links from Gramps as well, for example).
• If so, do we want such URLs to be able to determine whether the resource is static or localized?
• If we follow that train of thought some more: can we come up with a consistent way of determining whether resources are static or localized? Do we then still need separate URL generator interfaces? Do we then still need the static_url and localized_url Jinja2 filters?
• If so, we will probably want a base Link class/interface that ancestry and navigation links can extend.