* draft a download-api
* basic implementation
* allow partial downloads for protected chats
* use a separate column for download_state
* force a minimal timeout for delete_server_after in combination with partial messages
* add a warning if a possible download may expire by delete_server_after
* test load_imap_deletion_msgid()
* add a test for a partial download
* improve documentation and visibility
* let get_download_limit() return Result<Option>
* rusty getters
* apply MIN_DELETE_SERVER_AFTER to shown availability time
* move stub-creation to download.rs, use stock-strings, nicer logging
* make clippy happy (cargo clippy --tests)
* refine tests and comments
* fix typo
* remove superfluous closure in ffi
* respect partial_download for immediately scheduled DeleteMsgOnImap jobs