Render the shortcodes as late as possible

Render the shortcodes as late as possible

This is needed to make shortcode users happy with the new multilanguage support,
but it will also solve many other related posts about "stuff not available in the shortcode".

We will have to revisit this re the handler chain at some point, but that will be easier
now as the integration test story has improved so much.

As part of this commit, the site-building tests in page_test.go is refreshed, they now
tests for all the rendering engines (when available), and all of them now uses the
same code-path as used in production.

Fixes #1229
Fixes #2323
Fixes ##1076