FEATURE: Adds support for the WEBP image format (#11532)

Adds support for both still and animated webp images.
Animated images won't be optimized or get thumbnails.

Depends on https://github.com/discourse/discourse_docker/pull/506
and https://github.com/discourse/discourse_docker/pull/507

app/models/optimized_image.rb

@@ -183,7 +183,7 @@ class OptimizedImage < ActiveRecord::Base
     end
   end
 
-  IM_DECODERS ||= /\A(jpe?g|png|ico|gif)\z/i
+  IM_DECODERS ||= /\A(jpe?g|png|ico|gif|webp)\z/i
 
   def self.prepend_decoder!(path, ext_path = nil, opts = nil)
     opts ||= {}

config/site_settings.yml

@@ -1221,7 +1221,7 @@ files:
     list_type: compact
   authorized_extensions:
     client: true
-    default: "jpg|jpeg|png|gif|heic|heif"
+    default: "jpg|jpeg|png|gif|heic|heif|webp"
     refresh: true
     type: list
     list_type: compact