Webchat Improvements

[rockwellll]

• Prevents sending empty messages (no content & attachments)
• Prevent the bubble from showing up for attachment-only messages.
• Fixes an issue with subsequent attachment sending due to not resetting the file input after message delivery.
• use requestAnimationFrame for initial scroll to bottom when the webchat's previous state was opened which led to the container not scrolling to the bottom.
• Allow adding multiple attachments for a single message.
• Attachment remove and addition works now.