Adds "locked explanation" preview for locked files

fixes CNVS-17407

Test Plan:
* Enable "Better File Browsing" feature flag
1. Have a file that is restricted with the "UNTIL" date chosen to yesterday
2. Open the file as a teacher, copy the actual link to the file
3. As a student, try to access the file with the link
4. A preview should appear with a message stating that the file is locked

Change-Id: Ied97387c1d8a1d2c93f6c0cad5447d80523f4a24
Reviewed-on: https://gerrit.instructure.com/45826
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Dan Minkevitch <dan@instructure.com>