Chunk hack for jinja macro

[dzhuang]

Works only for contents in chunks.

Jinja2 macro in repo file macros.jinja.

{% macro show_if_after(datespec) -%}
{%- if now is defined and parse_date_spec(datespec) > now %}
<div markdown="block" class="hidden">
{%- elif now is defined and parse_date_spec(datespec) <= now %}
<div markdown="block" >
{% endif %}
{{ caller() }}
</div>
{%- endmacro %}

The macro should be imported with with context, i.e.:

{% from "macros.jinja" import show_if_after with context %}

[inducer]

As you can probably imagine, I'm not super-excited about this patch:

• First, there's no real reason why Jinja in page chunks should behave differently from the way it does everywhere else.
• Next, having to hand-write/import that macro every time seems kind of clunky.
• Lastly, this ignores caching and therefore will show the wrong content at times.

I'll leave this open, as I think there are pieces that are worth keeping.

[dzhuang]

Yes, you are right. Maybe the value of this PR is just to show what we can do if we can pass some markup context to the content rendered.