unlink: miscellaneous ameliorations

- More sensible dialog size
- Add a brief explanation at the top of the dialog
- Reason box becomes an input box (was a textarea)
- Add "Select/Deselect All" buttons
- Update default namespaces to include Template (for navboxes) and Draft

I was going to implement scrollbox here, but it turned out to be too
difficult.
morebits.css: remove now unused extraMenu CSS class

Doesn't do anything meaningful anymore.
twinkle.js: Make custom portletArea work properly

No idea if anyone uses this, but just in case...