2.1.0 - Silverstripe CMS 5 compatibility

And a general tidy-up of code, along with other dependency upgrades.

2.0.1 - fix PHP issuing E_DEPRECATED

FIX php depecation error for video slides

Error: E_DEPRECATED: preg_match(): Passing null to parameter #2
($subject) of type string is deprecated

promo-overlay/src/models/PromoSlide.php:121

If no video is set, do not try to parse it for a YouTube video ID.