Added content restrictions to sortby field #2033

Plugins can register 'sort' options with the new `sort_register()` function.  Originally I was planning on using any plugin's sort_* method as a sort, but then I have no label.  As long as I was adding a line to label each of them (which seems easy to forget) I figured just making it a function would be easier.

Just like with display styles, sort plugins send an array of type/bundle restrictions to Drupal.settings.  When the content type changes on the form, restricted elements (Event sort, year of publication, etc) are removed unless they're allowed.

These sorts exist for biblio and events.  I still have to add news and presentations.