Add support for IMAP folder subscriptions

[marclaporte]

IMAP folder subscriptions permits to choose which folders are visible in an email client. It's useful to temporarily hide unused folders. It is a standard, so generally supported by email clients: https://datatracker.ietf.org/doc/html/rfc3501#section-6.3.6

Let's review the UX of other popular email clients (both desktop and web-based) and make something nice.

Potential follow-up task: https://github.com/jasonmunro/cypht/issues/460