Allow to hide accounts from the unified inbox

[Ivan-Roger]

https://connect.mozilla.org/t5/ideas/ability-to-select-which-account-appear-in-the-unified-inbox/idi-p/82621#M44535

There is currently no way to select which account appear in the unified inbox.

This means that if you have many account you always have them all shown in the unified inbox.

This PR proposes a new option on accounts to allow hiding an account from the unified inbox.

[cketti]

Thank you for taking the time to work on fixing a user problem ❤️ However, I don't think we need a separate setting for this.

While it's technically true that a user can't configure which accounts are displayed in the unified inbox, it's something that can easily be accomplished without adding any additional code.
The unified inbox displays the contents of all folders (of all accounts) that have the "unify" folder setting enabled. By default the inbox of all accounts have this enabled. And so the contents of all inboxes appear in the unified inbox. However, this can easily be changed by going to Manage folders → Inbox of an account, then unchecking Unify.

[cketti]

Closing this since there's already a mechanism to exclude folders from the unified inbox.