Emoji fixes, optimizations and improvements

• Gradual loading of emoji to avoid halting browser thread while Vue renders tons of nodes.
• Limit emoji display to 500, add option to load all at user's own risk
  • had to add new styles for .alert.warning
• Not loading emoji before picker is open (closes #678 (closed))
• Show above input if not enough screen space (closes #674 (closed))
• Fix emoji loading on every login instead of first
• Fix space not being reserved for emoji putton in inputs (closes #682 (closed))
• Fix scrolling to the end of attachments if any instead to submit button (closes #686 (closed))