Kurento may *rarely* generate WebM/MKV files with corrupt or absent
SeekHead sectors. bbb-webrtc-recorder also doesn't generate SeekHead or
even the Cues sectors by default.
While those are are *optional* fields by spec, files need to be seekable
for our recording processing scripts to work.
This commit adds a remuxing step for Kurento and bbb-webrtc-recorder raw
files that is executed during the archive phase. It should re-include
any of the missing fields that make files seekable and restore the Cues
sector in WebM files.
bba4658dd3