[Shared Content] Preview doesn't work [DS-47]

[Shared Content] Preview doesn't work

(DS-47)

Steps to test:

[ ] As admin make sure Shared content modules are installed on e.g. Rose and Carnation themes
[ ] go to admin/virtual-y/shared-content/server on Carnation
[ ] add Rose environment as new source
[ ] make sure the token was generated
[ ] go to /admin/virtual-y/shared-content/shared_content_source on the Rose theme and check the Status checkbox
[ ] go to admin/virtual-y/shared-content/server on the Carnation theme and click the Fetch button
[ ] click the Preview button
[ ] you should see Popup with preview of the

Please check these boxes to confirm this PR covers the following cases:

Maintaining our upgrade path is essential. Check one or the other:
- [ ] This PR provides updates via hook_update_N or other means.
- [x] No updates are necessary for this change.
Front end fixes should be tested against all of the Open Y Themes.
- [ ] Tested against Carnation
- [ ] Tested against Lily
- [ ] Tested against Rose
- [x] This change does not contain front-end fixes.
[x] I have flagged this PR "Needs Review" or pinged the VY devs/QA team in Slack