Remove @locked_domain if it has been provided in the JID. (#1132)

Fixes #917.

src/converse-controlbox.js

@@ -471,7 +471,11 @@
 
                     let jid = form_data.get('jid');
                     if (_converse.locked_domain) {
-                        jid = Strophe.escapeNode(jid) + '@' + _converse.locked_domain;
+                        const last_part = '@' + _converse.locked_domain;
+                        if (jid.endsWith(last_part)) {
+                            jid = jid.substr(0, jid.length - last_part.length);
+                        }
+                        jid = Strophe.escapeNode(jid) + last_part;
                     } else if (_converse.default_domain && !_.includes(jid, '@')) {
                         jid = jid + '@' + _converse.default_domain;
                     }