Better styling of messages with mentions
The current styling of messages that mention the user isn't great: The name is not highlighted, the red color makes it seem like an error happened, and (some) color-blind people can't really see it at all.
I propose changing it to this:
- Change background color to faint blue
- Add 3px blue line on the left
- Highlight name in blue
- Add avatar next to the name (if it's too hard to do right now we can move it to a separate issue)
- Looks as expected