Configure ACLs to make it impossible to create new folders

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Dovecot supports ACLs: https://doc.dovecot.org/2.3/configuration_manual/acl/
We do not need IMAP extension, just mail_plugins = acl.

It should not be possible to create folders, also no need to write, only write-seen and write-deleted. Inserting is still needed until chatmail/core#7654 is used by most users as writing messages over IMAP was previously used to upload sync messages.

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Previously closed at #253, but for the reasons of tests. And as we want to get rid of folder handling (chatmail/core#7648), there will be no need for tests, we can also remove the tests for folders already.