Add explanatory message to logout button

[jessepollak]

When a user that logged in with Clef tries to log out via the WordPress web interface, we should pop up a modal telling them that they should log out on their phone.

This way, in the words of a confused user, "they get educated, AND they don't mistakenly think they're logged out (as I did)."

I really like this feature and think it will add a lot to the educate-while-doing paradigm we're trying to create.

[lolux]

It might be worthwhile to consider including the ability to dismiss the message permanently (or not showing it to admin-level users). WP admins who manage multiple single sites and/or WP network managers may actually prefer to stay logged in to the Clef App despite logging out of one or more WP sites.