feat(admin): category content on the Blog Editor and editor image uploads

- category content is the blogeditor type (uploadFolder categories): it is
  stripped and reported like post content; the 422 CheckHTML gate is
  retired and blank content still becomes NULL
- blogeditor and mlblogeditor Upload: magic-byte check for JPEG, PNG, GIF
  and WebP matching the extension, safe lowercase names, storage through
  classes.StoreMedia in the field's uploadFolder
- TestBlogEditorUploadSmoke, TestCategoriesAdminContent asserts the strip,
  README describes the field types, admin saves and editor uploads

models/category/fields.yaml

@@ -25,6 +25,7 @@ fields:
     content:
         label: golem15.journal::lang.category.content
         comment: golem15.journal::lang.category.content_comment
-        type: textarea
+        type: blogeditor
+        uploadFolder: categories
         size: huge
         span: full