Remove deaddrop chat

Contact request chats are not merged into a single virtual "deaddrop"
chat anymore. Instead, they are shown in the chatlist the same way as
other chats, but sending of messages to them is not allowed and MDNs
are not sent automatically until the chat is "accepted" by the user.

New API:
- dc_chat_is_contact_request(): returns true if chat is a contact
request.  In this case option to accept and block the chat via
dc_accept_chat() and dc_block_chat() should be shown in the UI.
- dc_accept_chat(): accept contact request and unblock the chat
- dc_block_chat(): decline contact request and block the chat

Removed API:
- dc_create_chat_by_msg_id(): deprecated 2021-02-07 in favor of
  dc_decide_on_contact_request()
- dc_marknoticed_contact(): deprecated 2021-02-07 in favor of
  dc_decide_on_contact_request()
- dc_decide_on_contact_request(): this call requires a message ID from
  deaddrop chat as input. As deaddrop chat is removed, this call can't
  be used anymore.
- dc_msg_get_real_chat_id(): use dc_msg_get_chat_id() instead, the
  only difference between these calls was in handling of deaddrop chat
- removed DC_CHAT_ID_DEADDROP and DC_STR_DEADDROP constants

src/constants.rs

@@ -25,7 +25,7 @@ pub static DC_VERSION_STR: Lazy<String> = Lazy::new(|| env!("CARGO_PKG_VERSION")
 pub enum Blocked {
     Not = 0,
     Manually = 1,
-    Deaddrop = 2,
+    Request = 2,
 }
 
 impl Default for Blocked {
@@ -123,8 +123,6 @@ pub const DC_RESEND_USER_AVATAR_DAYS: i64 = 14;
 // do not use too small value that will annoy users checking for nonexistant updates.
 pub const DC_OUTDATED_WARNING_DAYS: i64 = 365;
 
-/// virtual chat showing all messages belonging to chats flagged with chats.blocked=2
-pub const DC_CHAT_ID_DEADDROP: ChatId = ChatId::new(1);
 /// messages that should be deleted get this chat_id; the messages are deleted from the working thread later then. This is also needed as rfc724_mid should be preset as long as the message is not deleted on the server (otherwise it is downloaded again)
 pub const DC_CHAT_ID_TRASH: ChatId = ChatId::new(3);
 /// only an indicator in a chatlist
@@ -404,7 +402,7 @@ mod tests {
         assert_eq!(Blocked::Not, Blocked::default());
         assert_eq!(Blocked::Not, Blocked::from_i32(0).unwrap());
         assert_eq!(Blocked::Manually, Blocked::from_i32(1).unwrap());
-        assert_eq!(Blocked::Deaddrop, Blocked::from_i32(2).unwrap());
+        assert_eq!(Blocked::Request, Blocked::from_i32(2).unwrap());
     }
 
     #[test]