When processing custom emoji, ensure a non-animated version exists (#5230)

Use the non-animated version in web UI, but return both in API

app/models/custom_emoji.rb

@@ -21,7 +21,7 @@ class CustomEmoji < ApplicationRecord
     :(#{SHORTCODE_RE_FRAGMENT}):
     (?=[^[:alnum:]:]|$)/x
 
-  has_attached_file :image
+  has_attached_file :image, styles: { static: { format: 'png', convert_options: '-coalesce -strip' } }
 
   validates_attachment :image, content_type: { content_type: 'image/png' }, presence: true, size: { in: 0..50.kilobytes }
   validates :shortcode, uniqueness: { scope: :domain }, format: { with: /\A#{SHORTCODE_RE_FRAGMENT}\z/ }, length: { minimum: 2 }