Change `Artwork` to `Banner` in Show where its appropriate, make ban scope nullable because `site_wide` is a thing

LandingPage/models.py

@@ -66,7 +66,7 @@ class Show(TimestampedModel):
             storage=show_static_storage,
             upload_to = name_banner,
             help_text="A banner used for the show's page.",
-            verbose_name="Artwork"
+            verbose_name="Banner"
     ) 
     def __str__(self):
         return '%s [%s]'%(self.name,self.abbr)
@@ -132,6 +132,7 @@ class Ban(TimestampedModel):
     )
     scope = models.ManyToManyField(
             Show,
+            blank=True,
             help_text='The shows this user is banned from interacting with',
             verbose_name='Banned From',
             related_name='bans'