Add optional account overview screen

[cketti]

I don't intend for K-9 Mail to switch back to the old navigation model. However, adding an optional account overview screen should be possible without having to maintain two completely separate user interfaces. This screen will be disabled by default.

The initial plan is this: When enabled, K-9 Mail will start with an account overview screen, similar to that of K-9 Mail 5.600. There will be no side drawer in this screen. Tapping an account will open the auto-expand folder of that account, displaying the regular 5.8xx message list screen. The only difference will be that the account switcher in the drawer will be hidden. Users will have to go back to the account overview screen to switch to another account.

For a list of properties users valued in K-9 Mail 5.600's account overview screen see https://forum.k9mail.app/t/why-was-the-account-overview-screen-useful/1573

We use this issue tracker to keep track of the work that needs to be done. If you wish to discuss the account overview screen please use the forum. Locking this issue because in cases like this there's always a handful of people who believe their comments are so valuable they have to ignore the request to take discussions elsewhere.