report: Add video to the top of the testrun page

It looks great, makes triage easier and makes the awesome video recording
feature more visible.
report: Better quality of video thumbnail

This increases the size of the thumbnail so that text is readable, but
still keeps a low filesize to load quickly when navigating through lots
of results by pressing the up/down arrow keys (~12k for a screenshot of
some UI elements with a plain background, ~22k with TV in the
background).

This also preserves the aspect ratio of the source video.
release notes wording

This is slightly clearer that the video recording isn't new, only the
way we display it in the report.
report video thumbnail: Don't run `stbt screenshot` if we already have a screenshot
report video: Test that the video thumbnail is present in the report
report video thumbnail: Hide error in `stbt batch run` output

Avoid error in `stbt batch run` terminal output if `stbt screenshot`
failed for any reason, such as a NoVideo timeout.

Thanks to Lewis Haley for the suggestion.