Fix bug with import tiddler handling
Styling adjustments for the upgrade wizard
Stop defaulting to automatically updating the address bar
Add subtiddler support to the transclude widget

Useful to be able to reach into plugins. It will enable us to do things
like extract readmes from themes that are not active.
Add preview to upgrade wizard

The tiddlers are wikified in the context of the host wiki, which means
that mutual transclusions between imported tiddlers won’t appear
correct in the preview.