Reset submission number on clear

When clearing all webform results, it'd be nice to also have the option to reset the submission number from the same screen, rather than having to go separately back into the webform's advanced settings...

This is useful when clearing test data before making a webform live.

[bugfolder]

I would love to have this capability.

A complication is that there are many places from which one could clear a webform: on the webform's "Results" page, in Views of webforms that contain operations, etc. So I would suggest that this should be a single config setting for Webforms, i.e., a checkbox at admin/config/content/webform, in the "Advanced options" collapsed fieldset, whose effect is site-wide.

And then in webform_results_clear_form(), note that (if checked) the submission number will be reset, and in webform_results_clear_form_submit(), perform the resetting.