Joe's checklist of document viewer improvements

[hoyla]

• Use server-side events (SSE) to improve performance
• Better colour for focused highlight
• Keyboard navigation
• Our own loader icon, do not depend on semantic UI
• Pre-render pages with next/prev highlights (then could use scrollIntoView)
• Jump exactly to highlight (could be enabled by above)
• Do not go to server when search is emptied, tell PageCache to wipe out highlights without a round trip
• Exact searching: would we ever want ability to turn off tokenisation and emulate typing "find" functionality?
• White box for loading pages