Genericize implementing views which expose common attributes

Summary: Some views expose some/most/all of the basics (id, class, sigil, metadata, etc); let them extend a dedicated view to get it for free.

Test Plan: Viewed home page, paste, mobile dropdown, etc.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4440