feat: receive pre-messages and adapt download on demand

[Simon-Laux]

This PR implements receiving of pre-message, download of full messages and the new logic for deciding what to fetch when from IMAP.

This is part of #7367

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progress:

• changes to imap loop
• change download table to work with rfc724_mid instead of msg_id (added db migration for it)
• add a table for available_full_msgs
• move download logic from scheduler.rs to download.rs
• receive_imf
  • implement pre message
  • implement full message (if pre message exists then replace only attachment and change view type)
• re-check logic / self review
• write tests (recycle old tests that I removed in remove: partial downloads (remove creation of the stub messages) #7373 and write new ones)

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specific aspects that should become reality:

• In receive_imf, the full-message can be handled similarly to Chat-Edit (grep for "ChatEdit" in receive_imf.rs).
• As opposed to the current download-on-demand, we won't replace the whole msgs row, because this way, it will just work if an edit-message is received, and then later the user requests to download the attachment.
  • Need to change Viewtype, probably metadata params like image size
• pre-message gets Viewtype::Text in the database until downloaded
• background_fetch should not download messages with the Chat-Is-Full-Message header. If a large message without this flag is received, the UI should be notified via an event.
• A read receipt should be shown already when the user saw the pre-message.