New approach to widgets in series

Series can still have 1, 2 or 3 columns, but when they have multiple, instead of instantiating new regions, there are dropdowns from which to select from existing regions. The series template displays the sidebars selected.

Couple of notes on this:
- By default some sidebars are filtered out of the list, see line 144 of sidebars.php. There's also a filter there so that list of exlcuded regions can be modified.
- The "Advanced" Theme Options tab now has two new dropdowns allowing users to choose which sidebars should be used by default, which should make creating new series pages quicker.