Prompt users to configure special folders during account setup

[cketti]

If the IMAP server doesn't support the Special-Use extension to let clients know which folders to use as special folders (Archive, Drafts, Sent, Spam, Trash), prompt the user to configure special folders when setting up an account. The UI should be initialized with sensible defaults. If there's a folder called "Drafts" chances are good users will want to use it as their Drafts folder. Ideally, the app correctly guesses which folders to use for what role and the user only has to click an "OK" button.

Currently we default to not using a special folder when the server doesn't support the extension or no folder is tagged with a particular role. But most users don't expect e.g. sent messages not being uploaded automatically, especially if there's a folder called "Sent".