Improved Note Spacing

Notes now space their lines much as other Markdown apps and web pages do, which makes them more pleasant to read and their structure easier to follow.

• Two lines that both have text, such as two list items, now sit a little further apart than the lines of one wrapped line. An empty line still starts a new paragraph
• Headings get half a line more space above them, so they sit closer to the text they introduce
• Code blocks, dividers, videos, diagrams, tables of contents and embedded notes are spaced like lines of text, and a table has an empty line's space below it
  
  
• Line spacing now defaults to 1.4 instead of 1.2. Brains that have their own line spacing keep it
• Set the new "Line gap" in the brain's content settings, from 0% to 40% of a line (30% by default). At 0%, lines are spaced as before

• Thought cards, exports, note versions, Quick Thoughts and the backlinks and unlinked mentions lists follow the brain's spacing