Document view fixes

[taramk]

adjustments:

• cursor color on dark mode
• set line height to match standard nodes (24px/1.5rem)
• don’t allow indenting paragraphs
• disable checkboxes, clocks
• use --color-text-placeholder for markdown syntax (serves same purpose of being visible but as unobtrusive as possible)

bugs:

• command palette doesn't open when a document paragraph is selected (Unhandled Promise Rejection: TypeError: null is not an object (evaluating 't.editor'))

• pasting text into an empty document node - text disappears
• TypeError when putting cursor in document node parent header (Unhandled Promise Rejection: TypeError: null is not an object (evaluating 't.editor')). With Document View open, put the cursor in the Document title/header at the top of the page.
  now getting a slightly different message:

• select text + delete not working

chrome-specific issues:

• with the cursor at the end of a document paragraph node, hit enter and start typing: new paragraph is created but no text appears (https://share.getcloudapp.com/eDuEXWdO - this time it was extra weird, i was mashing my keyboard and it only recorded a few characters...)
• can't backspace through any document node text (when i tried this a second time, it put the cursor at the beginning of the node. so put the cursor anywhere in a document paragraph node, hit backspace once, and the cursor will go to the beginning of the node (nothing is deleted)

[progrium]

determined node header error is not specific to document view, but any node you've opened/zoomed. since its unrelated and non-trivial, it can be a new issue. closing this one and will leave #246 open until confirmed good to go (outside this last, separate issue)