Rewrite <video> tags and their inner tags

[benoit74]

In few pages of geo.libretexts.org (and probably other pages in other websites), we have HTML with <video> tags.

E.g. on page ID 13022 at https://geo.libretexts.org/Courses/Diablo_Valley_College/OCEAN-101%3A_Fundamentals_of_Oceanography_(Keddy)/11%3A_Ocean_Circulation/11.03%3A_Thermohaline_Circulation:

<video class="mt-media" controls="controls" preload="auto">
  <source
    src="https://svs.gsfc.nasa.gov/vis/a000000/a003600/a003658/thermohaline_conveyor_30fps.mp4"
    type="video/mp4"
  />
  <embed
    class="mt-media"
    src="https://svs.gsfc.nasa.gov/vis/a000000/a003600/a003658/thermohaline_conveyor_30fps.mp4"
    autoplay="False"
    autostart="False"
    scale="tofit"
    wmode="opaque"
    allowfullscreen="true"
  />
</video>

This needs to be rewritten to ensure we will not load the online resource. Probably a simple text indicating that there was a video here with the URL(s) of the source / embed is sufficient in a first iteration. Ideally we could image to grab the first still image of the video and push it to the ZIM, but it is far too complex.

Already replacing multiple HTML tags at once is not yet supported in current HTML rewriting logic. We will need to evolve the library to support this.