* js: add support for keydown events
This will modify the player behavior even if the player element is unfocused.
Based on the YouTube key bindings, allow to
- toggle playback with space and 'k' key
- increase and decrease player volume with up / down arrow key
- mute and unmute player with 'm' key
- jump forwards and backwards by 5 seconds with right / left arrow key
- jump forwards and backwards by 10 seconds with 'l' / 'j' key
- set video progress with number keys 0–9
- toggle captions with 'c' key
- toggle fullscreen mode with 'f' key
- play next video with 'N' key
- increase and decrease playback speed with '>' / '<' key
* js: remove unused dependency 'videojs.hotkeys.min.js'
Support for controlling the player volume by scrolling over it is
still retained by copying over the relevant code part from the
aforementioned library.
* Add prefers-color-scheme support
This should fix <https://github.com/omarroth/invidious/issues/559>.
The cookie storage format has been changed from boolean
("true"/"false") to tri-state ("dark"/"light"/""), so that users
without a cookie set will get dark mode if they have enabled the dark
theme in their operating system. The code for handling the cookie
state, along with the user's operating system theme, has been factored
out into a new function `update_mode`, which is called both at window
load and at the "storage" event listener, because the "storage" event
listener is only trigerred when a change is made to the localStorage
from another tab/window (for more info - see
<https://stackoverflow.com/a/4679754>).
