Need a clean visual way to know if a diplomacy session is open

[jwrober]

Is your feature request related to a problem? Please describe.
If you have an open diplomacy session with another player and leave the screen to look at or do something, there is no visual way to see on the top bar that there is a meeting still open that needs your attention.

Describe the solution you'd like
Could be as simple as having the nations view button flash/pulse. Or could add the flag of the nation you are conversing with to the nations view button. Or could add the thumbs up and down icons to the nations view button. Really anything to remind a player that there is a meeting open.

Describe alternatives you've considered
None

Additional context
PR #1419 fixed an issue around the text on the button, but left off fixing it.