Improve user operations

Resolve #2197
Resolve #3367

src/misc/acct/parse.ts

@@ -1,4 +1,5 @@
 export default (acct: string) => {
+	if (acct.startsWith('@')) acct = acct.substr(1);
 	const splitted = acct.split('@', 2);
 	return { username: splitted[0], host: splitted[1] || null };
 };