Avoid escaping href urls

Currently, the html files generated by zola contain several unnecessary HTML/XML encoding character /, found in _urls_ in <a ... href="_urls_">...</a>. This merge request restores these / to / by appending Tera filter safe to corresponding template expressions {{ ... }}.

Docs for filter safe:

• https://www.getzola.org/documentation/getting-started/overview/#templates
• https://tera.netlify.app/docs/#safe

Not sure if this should be merged on master or develop, as this change is (or should be) transparent to manual readers.

The three cases in _macros.html is, as I guess, inherited from juice hence I've opened similar PR to it. See https://github.com/huhu/juice/pull/11.