fix screenreader bug in “create theme” menu
fix screenreader bug in “create theme” menu

Fixes: CORE-2513

Test plan:
* In Safari with VoiceOver enabled, navigate to the Themes page of an 
  account
* Click on the +Theme button
* Move the screen reader focus up and down through the menu and note 
  that the "Create theme based on..." text is read for each option

Change-Id: I42d9481a81514666a2f51e5c0dd1733f3156e40c
Reviewed-on: https://gerrit.instructure.com/184805
Tested-by: Jenkins
Reviewed-by: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>
QA-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
Product-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>

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