Show offline messages upon opening

[davidmhewitt]

When opening the rewrite of mail, messages are not displayed in the list until the backend has checked for new messages and downloaded them. I feel as though it's better to show the messages we already have offline while that is happening in the background as it sometimes takes ~10 seconds to download new messages while you're sat looking at a blank window.

Evolution is able to do this.

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[danirabbit]

For clarity, we can now already load messages instantly offline, but when we are online it takes extra time to startup. So we should load the offline pool first whether we're online or not