Show estimated video transcoding time

I'm getting a lot of questions by uploaders that want to know the time until an upload of theirs is transcoded. Unfortunately, this is almost impossible to estimate, because there are a lot of factors, like:

- video length and bitrate
- server CPU speed
- enabled resolutions
- other videos in the queue

Instead, it would be nice if Peertube could display the estimated transcoding time, probably on the upload screen.

It would also make sense to show the total length of the transcoding queue separately, and inform the admin if it is unusually long.

Start by mapping the upload and transcoding workflow in the repository, including where queue status is exposed and where the upload screen and admin views are rendered. Clarify how estimates should account for the listed factors and define the separate queue-length and unusually-long-queue behaviors before implementation.

Written by the indexing model from the issue text.