Document IMAP-IDLE client settings

[taggart]

The IMAP standard supports an extension named "IDLE" where a client can keep a connection open to the server and when new mail arrives at the server it "pushes" a notification to the client that mail is waiting and it is automatically retrieved. This can speed up mail delivery quite a bit and also has the benefits of clients not needing to check mail as often and reduced resource usage on the server. Riseup's IMAP server already supports it and it works well and several clients support it. It would be nice to document on our help pages so users enable it. I think this means the following:

• it would be good to add something generic to https://riseup.net/en/email/clients
• Thunderbird supports it, document in https://riseup.net/en/email/clients/thunderbird
• iOS, does not
• MacOS Mail.app does,  hints https://www.lifewire.com/instant-updates-imap-idle-os-x-1172878
• K-9 does, hints https://www.skynetbb.com/email-setup/k9-mail
• Outlook didn't appear to as of Mar 2017
• offlineimap might (but we don't have a page for it)

[taggart]

• evolution seems to support it too https://help.gnome.org/users/evolution/stable/mail-send-and-receive-automatically.html.en
• claws does not http://www.thewildbeast.co.uk/claws-mail/bugzilla/show_bug.cgi?id=1408
• sylpheed does with a plugin https://github.com/clehner/sylpheed-imap-notify

[wanderer]

* offlineimap might (but we don't have a page for it)

some documetion for offlineimap https://github.com/OfflineIMAP/offlineimap/blob/master/offlineimap.conf#L1108-L1127

[taggart]

I had a need for this again today because I wanted to point some users at it.