Broad update, polish and tidying for Elements core (v2)

[adamjudd]

Details

A general update and clean-up for the whole elements repo is overdue as we've progressed with development. With the benefit of time and foresight, getting rid of some technical debt and bringing things up to date with our new knowledge will help keep everything clean and efficient.

A non-exhaustive list of action items:

• Remove old elements
• Update dependencies
• Simpler i18n approach
• Simpler theming approach
• Updates to core tech like Nucleon and Rumour
• Separate admin and portal elements
• Major changes to the portal element