Improvement: Add mentions

[clgeoio]

PR Monitor is great, but what might be extra nice would be a tab to track mentions in comments.
Sometimes I get mentioned in a PR I am not assigned to and a miss this mention in the slew of emails and noise.

[fwouts]

Sounds like a good feature to have.

What would you imagine that looking like? In particular:

• Would you like a notification once, or every time someone responds from then on?
• Would you want this to appear in your incoming PRs tab, or a new "Mentioned" tab, or nowhere?