Also use text editor (CKEditor) for "summary" of a text field

Problem/Motivation

The Summary part of the textfield should have a WYSIWYG Editor because the format applies to body and summary.

Proposed resolution

@todo
- Lock down the format for summary in the widget. (?)

Remaining tasks

1. Update this summary to accurately reflect the proposed change.
2. Fix the existing test failures.
3. Add new tests to cover the new functionality.
4. Move this back to 'Needs review'
5. Reviews/refinements.
6. RTBC.
7. Commit.

User interface changes

WYSIWYG editor available on the separate 'Summary' field on text areas that support them.

Before

After

API changes

TBD.

Data model changes

TBD, hopefully none.