app_filters: Render HTML to render Jinja2 syntax within Markdown macros.

If a Markdown macro contains Jinja2 template code, it isn't rendered
because render_markdown_path calls template.render on the including
.md file before the macro has been included. And then the including
.md file is converted to HTML. Therefore, the template code within
a Markdown macro (if any) never gets rendered and is returned as it is.

Now, after the source .md file is converted to HTML,
render_markdown_path renders the resulting HTML so that any template
code within included macros (if any) is finally rendered.

zerver/templatetags/app_filters.py

@@ -1,7 +1,7 @@
 from typing import Dict, Optional, Any
 
 from django.conf import settings
-from django.template import Library, loader
+from django.template import Library, loader, engines
 from django.utils.safestring import mark_safe
 from django.utils.lru_cache import lru_cache
 
@@ -82,7 +82,8 @@ def render_markdown_path(markdown_file_path, context=None):
     if context is None:
         context = {}
 
-    template = loader.get_template(markdown_file_path)
-    markdown_string = template.render(context)
+    jinja = engines['Jinja2']
+    markdown_string = jinja.env.loader.get_source(jinja.env, markdown_file_path)[0]
     html = md_engine.convert(markdown_string)
-    return mark_safe(html)
+    html_template = jinja.from_string(html)
+    return mark_safe(html_template.render(context))

zproject/jinja2/backends.py

@@ -46,6 +46,12 @@ class Jinja2(django_jinja2.Jinja2):
             six.reraise(TemplateSyntaxError, TemplateSyntaxError(exc.args),
                         sys.exc_info()[2])
 
+    def from_string(self, template_code):
+        # type: (str) -> Template
+        return Template(self.env.from_string(template_code),
+                        self.context_processors,
+                        self.debug)
+
 
 class Template(django_jinja2.Template):
         """Context processors aware Template.

zproject/settings.py

@@ -298,6 +298,7 @@ if PRODUCTION:
 
 TEMPLATES = [
     {
+        'NAME': 'Jinja2',
         'BACKEND': 'zproject.jinja2.backends.Jinja2',
         'DIRS': [
             os.path.join(DEPLOY_ROOT, 'templates'),