updated pdf viewer
BLD-970: Fix flaky test.
Merge pull request #2986 from edx/dcs/a11y-pdfreader

Accessible PDF textbook viewer
Merge pull request #3215 from edx/anton/fix-test-video-position

Video: Fix flaky test.
Add auto screenshot functionality.
Addressing comments.
Addressing comments.
Addressing comments.
Addressing comments.