Roll back #429364

Issue Summary

#429364: Excerpt textarea not compatible with wysiwyg module was implemented to address a UI need--to be able to attach a WYSIWYG editor to the teaser text area. Likely there are better ways to achieve this goal.

#429364: Excerpt textarea not compatible with wysiwyg module fundamentally changed excerpt from a simple UI enhancement to a set of major overrides, including separate storage of a format value for the teaser, making the teaser essentially a field in its own right.

This change makes the module much more intrusive and also significantly complicates the upgrade path.

Arguably, the core UI has improved to the point excerpt will no longer be necessary in D7.

If users want a teaser that has its own format, they can and should be encouraged to use a separate content module field. Excerpt looks to me to be really useful and viable only if it is a light UI enhancement.

Proposed approach:

* Roll back #429364: Excerpt textarea not compatible with wysiwyg module.
* Implement hidden dummy filter format field teaser field.
* In form_alter, set the default value of the teaser field filter format to that of the body field filter format.
* Attach a JS update handler to body filter format field, setting the teaser field format (and hence triggering WYSIWYG behaviour)

In sum: rather than having separate storage for the teaser filter format, the body field's value should be used, providing a clean upgrade path to D7.