[ie/facebook] fix: extract missing channel follower count

[Naylepsh]

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Extract rough followers count, possibly rounded down to the closest million / thousand depending on the number of followers the uploader has.

The follower count is not available in the video page thus additional request is required. The profile_url cannot be inferred from passed url nor existing uploader as that doesn't always correspond to the profile id in the profile url path, hence the additional response data parsing.

Fixes #

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