Add 'Raw Mode' toggle that displays text in the editor with a special style applied

Specifically, it will force ltr, monospace font and make the font bigger.
This helps previewing text/tags/placeholders in RTL languages, and also
eases reading in general when it comes to dealing with error checks
and finding issues.
Toggle 'Show Timeline' style instead of swapping labels

Instead of two labels (.js-show-timeline / .js-hide-timeline) use one to avoid
extra jumpiness and just toglle the 'selected' class on the item. This also
makes the underlying JS code a bit cleaner.
Add style toggling to 'Add Comment' button