Eliminate duplication in blurb admin.
Clarify _prepare method.

Long term, if we support configurable templates, this ad-hoc approach
should change completely.
Disable double-click to remove while editing.
Simplify category editing.

* Remove redundant divs
* Add a 'hidden' binding instead of negating the argument in the
  'visible' binding
* Only edit when clicking the name, not the whole header
* Remove the OK button, instead stop editing when losing focus