It was set to a fake value of 1000 previously in order to ensure there would be enough page buttons.
This was because two sequential requests are necessary (one to get the channel id corresponding to the custom url, another to get the number of videos from the "all uploaded videos" playlist, the url for which can be generated from the channel id).
Since Tor has a high latency, I thought at the time that this would be too slow, but in practice it's not too big of a deal.
Introduces cachetools dependency in order to cache the function which gets the number of videos.
The get_channel_id function has also been fixed since the ajax api seems to have been removed.
