feat(YALB-1425): disallow gif images

This modifies the shared field_media_image field to disallow gif files
across the site as a way to disallow animations.  The idea was that if
you are using a GIF, you probably are animating it.  If you are not,
then you probably want to use a PNG file.

web/profiles/custom/yalesites_profile/config/sync/field.field.media.image.field_media_image.yml

@@ -24,7 +24,7 @@ settings:
   handler: 'default:file'
   handler_settings: {  }
   file_directory: '[date:custom:Y]-[date:custom:m]'
-  file_extensions: 'png gif jpg jpeg'
+  file_extensions: 'png jpg jpeg'
   max_filesize: ''
   max_resolution: ''
   min_resolution: ''
@@ -33,7 +33,7 @@ settings:
   title_field: false
   title_field_required: false
   default_image:
-    uuid: null
+    uuid: ''
     alt: ''
     title: ''
     width: null