Add device message for users having mvbox_move enabled

[link2xt]

mvbox_move will be deleted in the future: #7648

We can already add a device message announcing the change for all mvbox_move users. It will include users of only_fetch_mvbox but no need to talk about this specifically, we can assume that only_fetch_mvbox users know what they are doing and write a message for users who don't use complicated setups with aliases and server-side rules moving messages based on "envelope from".

The message could be something like

You are using legacy option "Move automatically to DeltaChat Folder". This option is deprecated and is planned for deletion. We recommend to disable it. If having chat messages mixed into your Inbox is a problem, you can move to a separate mailbox by doing ...

Need to draft a message first.

[ell1e]

Is it helpful if I help drafting?

You are using the legacy option "Move automatically to DeltaChat Folder". We regret to inform you this option is deprecated and is scheduled for deletion in a few weeks, due to maintenance burden and rare use. We recommend to disable it. If having chat messages mixed into your Inbox is a problem, you can move to a separate mailbox by following this guide here: https://example.com/some/blog/post

I think having the migration guide be a blog post that is separate might help, so that it can be updated with corrections and new info even after the release has shipped.

I think it might also be a good idea to mention if "Only Fetch from DeltaChat folder" is going away (has that been decided by now?) so if that is the case, people don't go like "hah I can solve this without a separate mailbox!" and ignore that advice.

[link2xt]

Guide should be not on the blog so it does not arrive in the feed of users who don't need it. Could be FAQ entry or separate page.

[link2xt]

I think it might also be a good idea to mention if "Only Fetch from DeltaChat folder" is going away (has that been decided by now?)

I'm not sure it's going away, if I can move it to a separate transport option ("folder") and the only code needed is selecting a different folder instead of "INBOX", then I'll just do this silently so we don't have to deal with users who try to migrate from this setup and have all sort of problems:
https://support.delta.chat/t/it-may-be-unobvious-that-the-deltachat-folder-use-case-is-going-away/4495/31

For device message I'd just pretend this option does not exist, but it can be mentioned in the FAQ if we have to remove it.

[ell1e]

So basically it's dependent on whether you can figure out a good code layout? Fascinating 😮 in that case I agree with you and perhaps it's best to just not mention that option for now, until it's more clear whether it'll stay aruond or not.

[r10s]

Guide should be not on the blog so it does not arrive in the feed of users who don't need it. Could be FAQ entry or separate page.

similar to blog, we also do not want to have it in the FAQ, most users don't need it - so, a separate page, or, a post at https://support.delta.chat - that is easy to do, does not cost future maintenance and is editable. the forums is anyway full of ppl asking about how to do $things with keys, email, even gpg ...