Feature Request: Add Top Nav

Created by: BenMorganIO

While I was cruising about, I noticed that I wanted to return to the app, but that link wasn't in the admin. Also, sometimes when you're running a job, you get a dashboard to look at jobs. There's no link anywhere to find them. You're also logged in (most of the time) and there's no conventional logout button.

What I'm asking:

Basically add a nav to the top right near the search to logout if user_signed_in? is true and defined?. And maybe a link to main_app.root_url as well.

An API to add new links (such as giving users the ability to link to a CMS admin and/or a jobs dashboard possible) would be great. I know Solidus/Spree has this.