Skip mail folders that cannot hold messages #1942

Description

With "Read all folders" (readAllFolders) turned on, receiving mail from a Gmail account fails with folder cannot contain messages, and nothing is received. Gmail has a [Gmail] folder that holds [Gmail]/Spam, [Gmail]/Sent Mail and the other system folders but cannot hold messages itself, and receiving tries to open it as any other folder.

Steps to reproduce:

1. Add a Gmail account (imap.gmail.com, port 993, IMAPS, an app password) and turn on "Read all folders" for it.
2. Receive mail for the account.

Affects 6.3 and 7.0.

Reason

A Gmail mailbox cannot be received with "Read all folders" at all.

Fix

A folder that cannot hold messages is shown in the folder tree but not read, and its subfolders are read as usual.