Block-based editing theme

Investigate a block-based editing approach. This would align us with modern solutions like Gutenberg and Squarespace and co, opening up the door to easier-to-build, more beautiful content and themes.

This could also facilitate improvements to some of our integrations, like H5P – resolving, for example, MDL-59483.

The likely approach to this will be via a new "block based theme" in Moodle LMS. This theme will be in addition to the Boost theme.

While there will be many underlying changes required to implement this functionality, "exposing" it to the UI via a new theme will allow for organisations to better manage their migration path. It will also allow for easier iteration and testing of the new theme without changing/breaking existing functionality