[e.cash] Stealth theme video speed

Summary:
Adding some functionality to control background video playback speed based on theme specification.

I went for 0 for the stealth theme as I thought it looked better/more stealthy than a slowed down version, but open to other thoughts here

Test Plan:
npm run dev
Switch themes with the bottom right switch, the background video should stop playing in stealth theme. It should resume when theme is switched back
Adjust the videospeed value in the theme to differnt numbers. See if it still works and if you find any values more appealing
@bot preview-e.cash
See if it builds and runs correctly

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D13838