Spinner to indicate backend activity

Somewhat related to #287

EDS can be doing a lot of stuff in the background like checking for mail and pulling down new messages. We should have a spinner to signify that something is happening.

Furthermore, when adding an account for the first time, it can take 5+ minutes to pull down the summary info for all of the messages in a large mail account. I don't know how best to show this, but this is the sort of thing that needs a progress bar so there's at least a sense of how fast it's going.

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