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.
Merge branch 'report-html5-video'

* report-html5-video:
  report video thumbnail: Hide error in `stbt batch run` output
  report video: Test that the video thumbnail is present in the report
  report video thumbnail: Don't run `stbt screenshot` if we already have a screenshot
  release notes wording
  report: Better quality of video thumbnail
  report: Add video to the top of the testrun page
stbt-batch.d/templates/index.html: Remove unused code

Unused since b784a841.
stbt batch report: Ignore invalid "extra-columns" lines

These lines can be added by the user's classify or pre/post-run hooks,
so we should be robust to garbage.
stbt batch: summary report: List pass/fail/error counts per testcase

The code is ugly, but it works. I had initially tried to use
underscore.js's `_.groupBy` but I can't use underscore.js because of
https://github.com/asimihsan/tablequeryjs/issues/20
stbt batch: summary report: Hide subtotals by default; click to show