Refactor of left and right sidebars into PrimaryPanel and ContextualPanel in...

d.o issue: #3452314

Goals

1. Move the Frontend P.o.C closer to preliminary wireframes
2. Backfill mocked endpoints with real data
3. Continuous refactoring to improve code quality

Non-goals

• Final styling/UI
• Final UX
• Full page rendering within the preview
• Editing of components
• a11y concessions or considerations

MR Contents

1. Refactor of left and right sidebars into PrimaryPanel and ContextualPanel in line with wireframes.
   1. Moved tree view to left based on early Wireframes.
   2. Implemented basic tab navigation in the side panels.
   3. PrimaryPanel, on the left, contains the component list and tree view
   4. ContextualPanel, on the right, now opens when a component is selected and is passed basic context about the component
2. Lifted state for hovered and selected component to UI slide in the Redux state

   1. Tree view, preview and contextual panel all use the shared state to remain in sync - hovering a component in the tree will highlight it in the preview and vice-versa

      

3. Live SDC/Component list
   1. List in the primary panel now shows a real list of available SDC's fetched from the backend
4. SDC markup

   1. Dragging a component into the preview will now async fetch the HTML and insert it into the preview