Support fenced code blocks in explanations

[knatten]

Explanations are rendered in quiz/templatetags/quiz_extras.py, using the Markdown package. It looks like it can only render code blocks using indentation, not using fences ("```").

I had a quick look at the docs, and this package comes with some extensions, one of which is fenced code blocks. That one has a warning, though:

Warning Fenced Code Blocks are only supported at the document root level. Therefore, they cannot be nested inside lists or blockquotes. If you need to nest fenced code blocks, you may want to try the third party extension SuperFences instead.

So I think we need to look into using that extension, or using a different markdown renderer alltogether.

Alternatively, we could keep using indentation for code blocks, but I think people expect to be able to use fences. And since we don't have a good preview, people won't notice that it doesn't work, especially for porting efforts like #293.

When fixing this issue, we should also see if the solution we pick can fix #343.

[tocic]

I tried the following quick patch. Fenced code blocks are rendered correctly now, but the issues with extra newlines and comments are still there.

diff --git a/quiz/templatetags/quiz_extras.py b/quiz/templatetags/quiz_extras.py
index 99bed09..20cd738 100644
--- a/quiz/templatetags/quiz_extras.py
+++ b/quiz/templatetags/quiz_extras.py
@@ -41,7 +41,7 @@ def to_html(text, autoescape=None):
     return mark_safe(
         standard_ref(
             custom_linebreaks(
-                markdown.markdown(text))))
+                markdown.markdown(text, extensions=['pymdownx.superfences']))))
 
 
 @register.filter()
diff --git a/requirements.in b/requirements.in
index 7da8dd6..4fb8a02 100644
--- a/requirements.in
+++ b/requirements.in
@@ -1,5 +1,6 @@
 Django>=4.1,<5
 Markdown
+pymdown-extensions
 mock
 parameterized
 tweepy
diff --git a/requirements.txt b/requirements.txt
index 59c20e1..1afa779 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,5 +1,5 @@
 #
-# This file is autogenerated by pip-compile with Python 3.11
+# This file is autogenerated by pip-compile with Python 3.12
 # by the following command:
 #
 #    pip-compile requirements.in
@@ -27,7 +27,9 @@ idna==3.7
 lxml==5.2.2
     # via splinter
 markdown==3.6
-    # via -r requirements.in
+    # via
+    #   -r requirements.in
+    #   pymdown-extensions
 mock==5.1.0
     # via -r requirements.in
 oauthlib==3.2.2
@@ -42,10 +44,14 @@ pip-tools==7.4.1
     # via -r requirements.in
 pycodestyle==2.11.1
     # via autopep8
+pymdown-extensions==10.9
+    # via -r requirements.in
 pyproject-hooks==1.1.0
     # via
     #   build
     #   pip-tools
+pyyaml==6.0.1
+    # via pymdown-extensions
 requests==2.32.3
     # via
     #   requests-oauthlib