Fix markdown filter

Not sure how this was working before hand, but it was broken. This
change is cribbed from wcivf, and seems to work locally now.

polling_stations/settings/base.py

@@ -163,7 +163,6 @@
     "rest_framework.authtoken",
     "rest_framework_gis",
     "django_extensions",
-    "markdown",
     "corsheaders",
     "pipeline",
     "dc_signup_form",

polling_stations/templates/election_explainers.html

@@ -1,5 +1,5 @@
 {% load i18n %}
-{% load markdown %}
+{% load markdown_filter %}
 {% if request.brand == 'democracyclub' %}
     <div class="ds-card">
         <div class="ds-card-body">>

polling_stations/templates/fragments/cancelled_election.html

@@ -1,4 +1,5 @@
-{% load i18n_with_welsh markdown %}
+{% load i18n_with_welsh %}
+{% load markdown_filter %}
 {% if cancelled_election.metadata.cancelled_election.title %}
     <h2>{{ cancelled_election.metadata.cancelled_election.title }}</h2>
 {% else %}

polling_stations/templatetags/markdown_filter.py

@@ -0,0 +1,14 @@
+from django import template
+from django.utils.safestring import mark_safe
+
+import markdown
+
+register = template.Library()
+
+
+@register.filter(name="markdown")
+def markdown_filter(text):
+    return mark_safe(markdown.markdown(text))
+
+
+markdown_filter.is_safe = True