Move scraping helpers to sayit

Change scrapers to use the base classes which are now in the sayit repository (following https://github.com/mysociety/sayit/pull/286).

Also remove the philadelphia scraper, which will reappear in the sayit-philadelphia repo shortly.

This removes any caching from this scraper, as it is not using `self.requests`. Easiest just to import utils.CACHE_DIR here too and use it?