move cover image stuff to its own concern and add processing

app/models/concerns/group_cover_image.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+module GroupCoverImage
+  extend ActiveSupport::Concern
+  
+  LIMIT            = 4.megabytes
+  IMAGE_MIME_TYPES = ['image/jpeg', 'image/png', 'image/gif', 'image/webp'].freeze
+
+  class_methods do
+    def cover_image_styles(file)
+      styles = { original: { geometry: '1200x475#', file_geometry_parser: FastGeometryParser } }
+      styles[:static] = { geometry: '1200x475#', format: 'png', convert_options: '-coalesce', file_geometry_parser: FastGeometryParser } if file.content_type == 'image/gif'
+      styles
+    end
+
+    private :cover_image_styles
+  end
+
+  included do
+    has_attached_file :cover_image, styles: ->(f) { cover_image_styles(f) }, convert_options: { all: '-strip' }, processors: [:lazy_thumbnail]
+    validates_attachment_content_type :cover_image, content_type: IMAGE_MIME_TYPES
+    validates_attachment_size :cover_image, less_than: LIMIT
+    remotable_attachment :cover_image, LIMIT
+  end
+
+  def cover_image_original_url
+    cover_image.url(:original)
+  end
+
+  def cover_image_static_url
+    cover_image_content_type == 'image/gif' ? cover_image.url(:static) : cover_image_original_url
+  end
+end