Add image processing and generate blurhash for server thumbnail (#19348)

Remove separate server hero setting

db/migrate/20221012181003_add_blurhash_to_site_uploads.rb

@@ -0,0 +1,5 @@
+class AddBlurhashToSiteUploads < ActiveRecord::Migration[6.1]
+  def change
+    add_column :site_uploads, :blurhash, :string
+  end
+end