theme: implement portable linking #377
Description
Depending on the setting of uglyURLs linking to project pages require different URLSs.
This is bad because your users now need to know internals of your sites configuration when writing content.
So in terms of links we just treat a page as it would always be a branch bundle with uglyURLs=false (eg. your page creates a directory of its given name with a file ìndex.html` and all page resources inside of it).
This may get obsolete by Hugo itself: gohugoio/hugo#10585