$primaryForm in the control panel doesn't block additional submit requests made

Description

Primary Form in the control panel doesn't block additional submit requests made. This can result in multiple entries being created on first save if a save button is clicked more than once.

Steps to reproduce

1. Click 'New Entry' on the entries listing screen to get to a new entry edit screen
2. Click 'Save and Continue Editing' after all required fields are filled out
3. Click the 'Save' button before the last request has completed

Additional info

• Craft version: 2.6.2989
• PHP version: 7.0.10

Temporary Fix

The code below included from a plugin fixes the issue, although I am not sure if this would cause any unintended consequences and would probably want to be rolled into cp.js.

jQuery(function($){
    if( Craft && Craft.cp && Craft.cp.$primaryForm ){
        Craft.cp.$primaryForm.on('submit',function(ev){
            var $form = $(this);
            if( $form.hasClass('submitting') ){
                ev.preventDefault();
                return false;
            }
            $form.addClass('submitting');
            $form.find('button[type=submit],input[type=submit]').prop('disabled',true).addClass('disabled');
            $form.find('.submit.menubtn').addClass('disabled').blur().data('menubtn').disable();
        });
    }
});

Start by examining cp.js and the Craft.cp.$primaryForm submit handling described in the issue. Reproduce the control-panel entry flow by using “Save and Continue Editing” followed quickly by “Save”; done means the second request is prevented and submit controls are disabled while the first request is pending.