Notification center for pings/mentions

Multiple other chat apps have `@` button somewhere for showing when you or `@here` have been mentioned and allowing quick jumping to those messages.

I am missing this feature in Keybase desktop, the most recent case is the `@keybasefriends#general` testing which was at 04:32 at night for me and there was a lot of chatting after that, so I had to scroll up a lot and see loading indicator for two or three times before seeing that it wasn't even to me personally.

If there was a mention/ping/notification/something center, I hope I could just be able to click it and see the notification and if I wanted to, click it to jump up in the chat where the notification actually happened.

No files, tests, or entry points are named. The requested notification center should show personal and @here mentions and let users jump to each originating chat message; completion would require an agreed desktop design and behavior.

Written by the indexing model from the issue text.