Use file_validators instead of Papeclip validations

Since we're going to remove Paperclip and Active Storage doesn't provide
any validations, we have to either write our own validation rules or use
a different gem.

We're using the file_validators gem instead of the
`active_storage_validations` gem because the latter doesn't support
proc/lambda objects in size and content type definitions. We need to use
them because in our case these values depend on settings stored in the
database.

app/models/site_customization/image.rb

@@ -14,7 +14,9 @@ class SiteCustomization::Image < ApplicationRecord
   has_attachment :image
 
   validates :name, presence: true, uniqueness: true, inclusion: { in: VALID_IMAGES.keys }
-  validates_attachment_content_type :image, content_type: ["image/png", "image/jpeg"]
+  do_not_validate_attachment_file_type :image
+  validates :image, file_content_type: { allow: ["image/png", "image/jpeg"] }
+
   validate :check_image
 
   def self.all_images