VideoBlockEditor:Poster Modal

Build 1554 on main
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Reviewing disabled
Baseline History
Build 1554139e868 current baseline
Current build. Accepted by Siyang Cao.
Build 15493368a29
Ancestor build. There were unaccepted changes, so this was not used as a baseline.
Build 15396c25067
Accepted by Siyang Cao.
Build 1521be7d9b9
Accepted by Tataihono Nikora.
Commenting is disabled because there's a newer build on this branch