Invalidate Block/Region cache when updating items in the region

Problem/Motivation

New Announcements don't appear right away when pages are cached. We need to force regeneration of the blocks when announcements are updated.

Steps to reproduce

Add a new announcement with caching enabled.

Proposed resolution

Ensure that cached blocks are tagged with the correct tags (regions and entity ids). When saving an announcement, ensure that the region's tag is invalidated along with the announcement.