Stop using IMAP APPEND to upload sync messages

[link2xt]

We need to revert changes done in #5901

These changes result in maintaining two IMAP connections that we want to reduce to one (#7648) and are not compatible with multi-transport as we want to send sync messages once to all our relays (see #7640 (comment)) instead of uploading to each relay or only to one relay.

After this change IMAP loop for mvbox should only be created if mvbox_move (or only_fetch_mvbox which depends on mvbox_move) is enabled. Users with default settings will have only one IMAP loop regardless of whether is_chatmail is true.

[iequidoo]

Just in case, that change doesn't result in two IMAP connections because everything is done from the Inbox loop: https://github.com/chatmail/core/pull/5901/files#diff-3d8724b3c76e2e43e07e0dcc66176a94d7735fc7736cd11bc8f4b00da454a164. Anyway, agree, it should be reverted

[link2xt]

Just in case, that change doesn't result in two IMAP connections because everything is done from the Inbox loop

Uploading is done from the inbox loop maybe, but then you still need to watch mvbox because this is where sync messages end up:

core/src/config.rs

Line 598 in 294e23d

|| !self.get_config_bool(Config::IsChatmail).await?)

If sync messages stay in the inbox, then we don't need to watch mvbox as well.