Provide option to turn on/off lazy loading

[JamesDPC]

The template Iframe.ss by default adds loading="lazy". It would be useful to be able to turn this off on a per-video basis.

AC

• add Boolean DB field to relevant models to turn off lazy loading (by default it can be set to enabled to preserve BC) e.g NotLazyLoad
• provide a method WillLazyLoad return bool, returns true when the record has lazy loading enabled
• provide a PolyFill() method returning an empty string to block loading of the loading attribute polyfill JS