Refactor table-of-contents view

The table-of-contents (TOC) view currently has the following drawbacks:

• non-intuitive: Wider screens show it next to the content, smaller screens hide it but show a button at the bottom
• very basic design: The 1.x version had more aesthetic appeal, we should improve this
• less features: In 1.x it tracks the scroll position and highlights the section where you are currently, this should be re-added

The floating TOC button at the bottom should be removed. The dynamic TOC display next to the content should be removed.

There should be a new button in the editor-bar that toggles the TOC aside of the content, regardless of the screen size.

Since the read-only-view (/s/:noteId) has no editor-bar, the TOC aside of the content should be shown there always.