Enable form data UTF-8 sanitisation

See:
* https://github.com/whitequark/rack-utf8_sanitizer/blob/b069c9ef6d69e58695ec9a7cad68c41b57812468/lib/rack/utf8_sanitizer.rb#L21
* https://github.com/whitequark/rack-utf8_sanitizer/blob/b069c9ef6d69e58695ec9a7cad68c41b57812468/lib/rack/utf8_sanitizer.rb#L84
* https://github.com/whitequark/rack-utf8_sanitizer/blob/b069c9ef6d69e58695ec9a7cad68c41b57812468/lib/rack/utf8_sanitizer.rb#L121

Collections currently do not have any POST/PATCH/PUT routes for uploading form data, but
this is being added for consistency with other repositories and in case
such routes will be added in the future.

config/application.rb

@@ -130,7 +130,6 @@ class Application < Rails::Application
     config.middleware.insert_before(
       0,
       Rack::UTF8Sanitizer,
-      sanitizable_content_types: [],
       only: %w[QUERY_STRING],
       strategy: Sanitiser::Strategy,
     )

lib/sanitiser/strategy.rb

@@ -11,7 +11,7 @@ def call(input, sanitize_null_bytes: false)
           raise NullByteInString
         end
       rescue StandardError
-        raise SanitisingError, "Non-UTF-8 (or null) character in the query or in the cookie"
+        raise SanitisingError, "Non-UTF-8 (or null) character in the query, cookie or form data"
       end
     end
   end