feat: organisation logo (attachOne logo) and payload helpers

- Organisation gains the public attachOne logo (OrganisationLogoField),
  editable as an image upload on the Organisations form; avatar unchanged
- OrganisationLogoURL and OrganisationLogoFields give applications the
  has_logo, logo and logo_url keys for their organisation payloads
- no migration: attachments live in system_files

models/organisation/fields.yaml

@@ -22,6 +22,13 @@ fields:
         mode: image
         imageWidth: 120
         imageHeight: 120
+    logo:
+        label: golem15.user::lang.organisation.logo
+        comment: golem15.user::lang.organisation.logo_comment
+        type: fileupload
+        mode: image
+        imageWidth: 240
+        imageHeight: 120
     members:
         type: relation-manager
         relation: members