[FR] Auto-subscribe to folders

[3nprob]

🚀 Feature

Use-case: There is a sieve on the IMAP server which dynamically creates new folders for incoming mail. These new folders should be included in "combined views" in order to not miss unread messages.

I can see this working several ways (either could be behind a feature config flag):

1. Auto-include folders in combined views according to IMAP subscriptions (preferred as it will align with other clients)
2. Folder-level setting in cypht to auto-include any subfolder for that folder

[marclaporte]

Related: https://github.com/jasonmunro/cypht/issues/9