since signing up through the big button on the welcome message leads to
/data_control, it needs a message on what to do next. the most common
option here is importing subscriptions from youtube, hence it was moved
to the top.
the rather inconspicuous link to google's support pages was replaced by
a infobox that instructs the user to click a link to download a file and
upload it on this page.
* 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>).
