Add check to prevent completely blank posts

This isn't perfect, but if a user tries to make a completely blank post,
they will be blocked from doing so

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

board/models.py

@@ -143,6 +143,10 @@ class Post(models.Model):
             )
 
     def clean(self):
+        # Make sure there is at least some content
+        self.text = self.text.strip()
+        if not (self.text or self.image):
+            raise ValidationError("Please either write a message or upload an image")
         # Image upload size check
         if self.image and self.image.size > settings.MAX_UPLOAD_SIZE:
             raise ValidationError(