Mute thread functionality like in thunderbird

Is your feature request related to a problem? Please describe.
Being subscribed to some medium level maling-lists it is sometimes frustrating when you know that a) a thread does not interest you in the slightest, b) there will be many more replies to it, that you can't do anything about this. In thunderbird you can mute that one thread (k for kill, see here) and still get notifications and unread emails for other threads from that list.

Describe the solution you'd like
Having an option in K-9 which can mute a thread. This would (to be discussed) have the following result:

• All emails currently in this thread are marked as read
• The thread is never shown as unread in the message list (i.e. automatically mark new incoming emails as read?)
• No notifications for new emails in said thread are generated
• The thread gets a special icon indicating it as ignored (thunderbid uses this:
  )
• It gets an option to unignore the thread, which would enable it for notifications and unread statuses again

Describe alternatives you've considered
Not really any idea.

Additional context
It would be super nice if this ignored thread status would sync via imap to other clients, especially that it works interoperable with TB. I don't know if Thunderbird currently even syncs this to the server though. But maybe we can request that as a feature there as well :-).

Some related work:

• https://github.com/k9mail/k-9/pull/5114 (abandoned but also slightly different scope)
• https://github.com/k9mail/k-9/issues/4330 (for hiding messages from ignored threads altogether, related but distinct functionality)

Start by reviewing pull request #5114 and issues #4330 and #5601 to understand the related and abandoned work. Compare the requested behavior for marking threads read, suppressing notifications, showing an ignored indicator, unignoring, and possible IMAP synchronization. Done means the agreed mute and unmute behavior is implemented without conflating it with hiding messages.