Draft: Reactify sidebar & footer

Detail

This is a

• bug fix
• feature
• refactor

This patch refactors the sidebar and header areas into React/TS. It moves a bunch of 'glue' code into TS to be able to do so.

To do before review:

• fix measures total value display in header
• fix scrolling behaviour on new page
• sort out '404' handling all the way through the page manager
• move legacy vars setting to legacy code
• move project/scenario manipulation code out of editor-sidebar

Checklist

• Manual test if possible
• Documentation impact considered
• Assign related ticket to reviewer

Related to #842 (closed)