webstaff: teach MARC editor some more tricks

webstaff: teach MARC editor some more tricks

[1] Now accepts a flat-only attribute specifying that
    the editor should be restricted to flat mode.
[2] Now accepts an embedded attribute specifying that
    various stuff that isn't useful when the editor
    is embedded in another control should be ignored.
[3] The width of the flat editor's textarea is now
    bounded by its container.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Kathy Lussier <klussier@masslnc.org>