ensure there is a default eportfolio page

fixes CNVS-14580

test plan:
1. create an eportfolio
2. click organize/manage pages
3. create a second page
4. open the eportfolio in a new tab
5. in one tab, delete the welcome page
6. in the second tab, delete the new page
7. refresh the eportfolio
8. it should still load the welcome page (having recreated it)

Change-Id: I400b2d4689773a8f48d8b3bfa577982492a4bfb4
Reviewed-on: https://gerrit.instructure.com/38847
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>