Auto-create summary sheets from preferences view

The gradebook preferences view needs a new fieldset titled 'Summary' which has the following explanation of what its button will do:

Click the following button if you wish to create summary worksheets in all of your classes.

If there already exists a summary worksheet in one of the classes, then the following paragraph and checkbox should appear before the button:

You already have a worksheet titled 'Summary' in at least one of your gradebooks. Click the following checkboz before clicking the button, if you'd like the existing summary worksheets to be overwirtten.

_ overwirte existing worksheets titled 'Summary'

The button is titled 'Create Summary Worksheets'