Remove setting 'sync server deletions'

Remove the setting 'Sync server deletions' and always delete the local copy when a message has been removed from the server.

Since this is a behavior change for people who don't use the default value where server deletions are synced, we need to inform those users about the change. I propose to disable automatic sync (and Push) and to show a warning message in the message list screen above the message list. Clicking this warning will display a dialog with more details. Acknowledging this message will restore the previous automatic sync and Push settings.
Manually refreshing a folder should probably also display this dialog if the change hasn't been acknowledged by the user yet.

This is part of the effort to remove support for local messages.

No files or tests are named. Start by reviewing the existing “Sync server deletions” setting, the message list screen, and the manual folder refresh flow, then read issue 4970 for context. Done means the setting is removed, server-deleted messages are always removed locally, and the proposed warning and settings-restoration behavior is implemented.