Download all bookmarked videos, or download all videos from channel

[WallGeckoPoint]

Is your feature request related to a problem? Please describe.
The best time to watch videos is often when you can't do anything else -- like when you don't have signal. Therefore, having all your bookmarked videos downloaded for later offline viewing is really useful.

Unfortunately, downloading videos one-by-one is time consuming and clunky. If you tap in the slightly wrong spot, you lose your position and get sent to the channel which posted that video.

Describe the solution you'd like
It would be much better to have an control which allows you to download all bookmarked videos at once. If some videos are already downloaded, this should not download them again.

Additionally, it would be useful to extend this control so that it can also be used to download every video from a given channel.

UI-wise, this control probably belongs in the "three dots" drop-down at the top right of the screen.

Describe alternatives you've considered
There aren't any bulk download options that I can find, so there aren't any alternatives.

Additional context
N/A