Add toggle to bubble Starred messages to the top

FastMail, Gmail, and Inbox have this rad feature where starred (in both their parlance, "pinned") emails can bubble to the top of the email list. I've been using this extensively and it'd be great to get that into Mail.


@snwh pointed out that Inbox also has this feature, also called "pinning", but it uses the same mechanism as starring under the hood.

> In terms of metaphorizing this behaviour, I'd be more inclined to call it "pinning" and drop the whole stars thing from Mail. Pinning implies (to me) something more immovable –e.g. a pinned Tweet or social media post at the top of your feed. - @snwh


From Launchpad [bug 1590976](https://bugs.launchpad.net/pantheon-mail/+bug/1590976).

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