Improve messages formatting

[yannickglt]

• [ ] Replace user/channel references with their name (e.g.: hi <@U02DSC9DX>)
• [ ] Replace links to messages with the message content (with a link to open it in the browser on enter key)
• [ ] Replace external links in order to open them in the browser on enter key
• [ ] Replace images with the image as icon and a link to open it in the browser