Remove empty HTML info-slide if nothing inputted

This commit adds a cleaning method for motions, so that if no info-slide
has been added, or it was removed, the info-slide re-becomes None rather
than something like "`<p><br></p>`".

tabbycat/motions/models.py

@@ -1,5 +1,6 @@
 from django.db import models
 from django.utils.translation import gettext_lazy as _
+from html2text import html2text
 
 
 class Motion(models.Model):
@@ -27,6 +28,11 @@ class Meta:
     def __str__(self):
         return self.text
 
+    def clean_fields(self, exclude=None):
+        super().clean_fields(exclude=exclude)
+        if not html2text(self.info_slide or '').isspace() and 'info_slide' not in exclude:
+            self.info_slide = None
+
 
 class DebateTeamMotionPreference(models.Model):
     """Represents a motion preference submitted by a debate team."""