fix(twitter): fixes Twitter API to work with 1.9 session

This updates the session handling to use the new elgg_get_session, uses set/get/remove rather than the deprecated array access, and cleans up ElggSession a bit.

Fixes #6917
The problem was that the global $SESSION is not actually an ElggSession, but a deprecation wrapper around it. The [wrapper's offsetGet](https://github.com/Elgg/Elgg/blob/1.9/engine/classes/Elgg/DeprecationWrapper.php#L147) incorrectly assumes that if the wrapped thing is an object, it can just grab the property.