Add poster frame attribute to video() helper and use the video() helper across bedrock

Add poster frame attribute to video() helper ([Bug 905347](https://bugzilla.mozilla.org/show_bug.cgi?id=905347)) and;
Use video() helper for videos across bedrock ([Bug 905348](https://bugzilla.mozilla.org/show_bug.cgi?id=905348))

@l-hedgehog: This should replace PR #1108.