Add relative_media_url and relative_content_url functions about hyde

[navilan]

For now, moving this out of this release - this is better suited for a version that focuses on multi-language features.

[nud]

I don't know if backward compat is an issue, but it could be implemented this way while keeping only media_url() and not adding rel_media_url():

• Consider the url to be absolute if it starts with a '/' (ie the same behaviour as media_url does now)
• Consider the url to be relative to the current document if it starts with something else.

The issue I can see here is: how should the relative path behave when used in a layout? Presumably in this case the relative media url should be relative to the resource.

It would also be possible to add an argument to media_url and content_url saying "relative to what?": media_url(resource_path, relative_to) with relative_to being 'self' (ie the current template) or 'resource'