Use abstract ModelWithImageFields for Show model

this renames the fields
- `ppoi` -> `image_ppoi`
- `width` -> `image_width`
- `height` -> `image_height`
and adds the fields `image_credits` and `image_alt_text`.

program/models.py

@@ -203,7 +203,7 @@ class HostLink(Link):
     host = models.ForeignKey(Host, on_delete=models.CASCADE, related_name="links")
 
 
-class Show(models.Model):
+class Show(ModelWithImageFields):
     predecessor = models.ForeignKey(
         "self",
         blank=True,
@@ -229,17 +229,6 @@ class Show(models.Model):
     music_focus = models.ManyToManyField(MusicFocus, blank=True, related_name="shows")
     name = models.CharField(max_length=255)
     slug = models.CharField(max_length=255, unique=True)
-    ppoi = PPOIField()
-    height = models.PositiveIntegerField(blank=True, null=True, editable=False)
-    width = models.PositiveIntegerField(blank=True, null=True, editable=False)
-    image = VersatileImageField(
-        blank=True,
-        null=True,
-        upload_to="show_images",
-        width_field="width",
-        height_field="height",
-        ppoi_field="ppoi",
-    )
     logo = models.ImageField(blank=True, null=True, upload_to="show_images")
     short_description = models.TextField()
     description = models.TextField(blank=True, null=True)