replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated
Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

Fix DOM XSS

fix fullscreen issues; correct size in Safari, auto-focus presentation when entering fullscreen #3080

Fixing background video not playing on certain small factor devices (android)

fix npm start -- --host=0.0.0.0 unuseful