Add "Keep conversations archived" option

[naveensingh]

Checklist

• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the request will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.

Feature description

Add an option to keep conversations archived.

By default, conversations are automatically unarchived (as expected) when there's a new message.

Why do you want this feature?

• Helps keep the main page nice and clean.
• There are senders I don't often interact with but they send non-essential updates that I want to check from time to time.

Additional information

• Related issues:
  • Hide muted conversations #333
  • Add "Mute archived conversations" option #335
• Conflicting(?) issue: Unarchive a discussion when prepare an answer #124