Add analogue of `__tera_context` but without content

Debugging in Zola using __tera_context is super convenient, I even created a special section on my site to put debug information.

But it’s only usable when a blog is small and there are not many articles. When I added all my articles, the browser becomes very slow, because __tera_context contained all the content information.

So, I suggest adding the new variable __tera_context_stripped without content. Or with the first 50 symbols of content or so.