Ensure filing's have access to a translator, regardless of whether it was initialized with one

Changed Fech::Filing#translator from being an attr_accessor to a memoization method. This ensure that a Translator is available to all filings by calling #translator on a Filing instance, even if one wasn't created when the Filing was instantiated.

This guards against a bug where a Filing is initially created without any default translations, but one later attempts to add an ad hoc translation by calling `filing#translate`, or by accessing the translator directly with `filing#translator`.