MemoServ: add IGNORE

IGNORE ADD/DEL/LIST manages a per-account memo-ignore list; a memo from
an ignored account is silently dropped, and the sender is still told it
was sent so the ignore isn't revealed. New memo_ignore account field +
events, and a check in SEND.

src/engine/db/mod.rs

@@ -79,6 +79,9 @@ pub struct Account {
     // Memos left for this account (MemoServ), oldest first.
     #[serde(default)]
     pub memos: Vec<Memo>,
+    // Accounts this user won't receive memos from (MemoServ IGNORE).
+    #[serde(default)]
+    pub memo_ignore: Vec<String>,
     // Personal greet a bot shows when this account joins a greet-enabled channel.
     #[serde(default)]
     pub greet: String,