Revamp commenting UI/UX

This attempts to freshen up the commenting UI/UX while also addressing #326.

• Tree structure for comment UI, so users will mistake less with making answers instead of comments. Implemented with CSS ::before and ::after on comments.
• Comment button both at top and bottom. Bottom one is subtle, only shows if there are >1 comments, and is on same line if there is already "show more comments" text.
• Save buttons moved to far right as the main action button. Delete Draft colours more subtle.
• Editor shadows removed since they often caused "double-shadow"