fix enabling the add module item button
fix enabling the add module item button

only check for a selection if the user is looking at a page
that requires a selection

test plan:
 - enable the module file drag and drop option
 - add a text header, external URL, or external tool to a module
 - add another one. the button should be enabled

fixes LS-1089

flag = module_dnd

Change-Id: Ib0a1d386a6768a287351d8aff9c8d713e9bb1886
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/242807
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: Robin Kuss <0b206b54367b2de0334896ea45826f0e3194bd35@instructure.com>
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms