Reduce duplicate code across plugins using notify popup

[kirsty-hames]

Subject of the enhancement

As per adaptlearning/adapt-contrib-vanilla#443, the default style for Notify icon buttons is consistent across all plugins however we've duplicated the styling code across the various plugin LESS files in Vanilla.

See Hotgraphic close button in comparison to core Notify close button. The CSS being applied is the same but attached to different selectors.

Hotgraphic targets .hotgraphic-popup__controls and .hotgraphic-popup__close

Suggest applying .notify__btn-icon to inherit from Vanilla notify.less and remove the duplicate code from Vanilla hotgraphicPopup.less. This means the default style is set in one place but we still have plugin specific classes to target should you want to style these different - although there's good reason for keeping UI button styling consistent.

[kirsty-hames]

To be resolved in Vanilla as per adaptlearning/adapt-contrib-vanilla#443 (comment)