download on demand (#2631)

* 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

src/context.rs

@@ -371,6 +371,12 @@ impl Context {
             "show_emails",
             self.get_config_int(Config::ShowEmails).await?.to_string(),
         );
+        res.insert(
+            "download_limit",
+            self.get_config_int(Config::DownloadLimit)
+                .await?
+                .to_string(),
+        );
         res.insert("inbox_watch", inbox_watch.to_string());
         res.insert("sentbox_watch", sentbox_watch.to_string());
         res.insert("mvbox_watch", mvbox_watch.to_string());