We lost the reporting of connection errors in the UI. These were
originally displayed in FolderMessageList, under the name of each folder.
In the new implementation, we borrow the "undo" banner from Gmail and
display it whenever there is a connection error in a mailbox.
* Add banner, and code to animate it on/off the top of the list.
* Toggle banner whenever a connection state change occurs in MessageList
* Toggle banner whenever a connection state change occurs in MailboxList
* Slight change to callback semantics for sendMailCallback
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