Add video file format icon to main page

[rosanna-smith]

It looks like we don't have an icon for video files on the main page like we do for audio, pdf etc.

For example, if I filter for mp4, the results look like this:

This one is similar to the icons we already have if we don't yet have a video icon: https://fontawesome.com/icons/file-video?f=classic&s=solid

[rosanna-smith]

Added to https://github.com/Language-Research-Technology/oni-ui/issues/36