Don't trigger 'wikipage.diff' hook for content that looks like a diff

Don't trigger 'wikipage.diff' hook for content that looks like a diff

Follows-up 9cfb9cb9fba, and b62f0e91564.

Add data-mw="interface" to elements created by the interface
(e.g. not user-generated content) and use this to narrow down
scope of elements eligible for JavaScript binding.

This avoids bugs where e.g. the diff hook triggers on a wiki page about diffs.
This isn't a security issue per-se, but causing odd behaviour.

Also add missing tests for data-ooui filtering (follows-up aa9a52da).

Change-Id: I9a0c86c92d411538bd9e203ec6ae54616fdf49b8