Gives the edit/add module form a better focus target

Because of the way the events are done/handled by the form
a submit occurs on the form whenever a form element has focus.
When the form dialog opens the first focusable item is given
focus.  This provides a empty non-interactive element to receive
focus when the form dialog opens preventing an automatic
submit from triggering.  While not ideal, it works.

fixes CNVS-14966

Test Plan:
  - Go to the modules page (/courses/##/modules)
  - Use the keyboard to navigate to the edit option in the cog
    menu.
  - Press "Enter" on the "Edit" option.
  - The form should not close automatically.
  - Do the same with module items.

Change-Id: Icf22c23b502b66a7678111224bf0907876bc72f8
Reviewed-on: https://gerrit.instructure.com/41031
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>