Add seo url for webm videos to circumvent a safari issue

[rs-amp]

Some versions of safari are unable to play webm videos if the url does not end in a .webm extension. With videos served by amplience, this issue was partially present, as the video would fail to load normally, but if you had visited the video before and cached the 302, it would work (as the redirect target has the extension .webm).

This PR adds a step after fetching set data where it enriches webm videos in the set to add an seo url, in the format profile.webm. This seems to allow the video to load on safari without needing to visit and cache the 302 redirect.