Use alternate MSISDN submit URL when returned by HS

This changes MSISDN token submission to send to an arbitrary URL (instead of the
current IS) when the HS provides such a URL.

Fixes https://github.com/vector-im/riot-web/issues/10923

src/AddThreepid.js

@@ -35,6 +35,8 @@ import IdentityAuthClient from './IdentityAuthClient';
 export default class AddThreepid {
     constructor() {
         this.clientSecret = MatrixClientPeg.get().generateClientSecret();
+        this.sessionId = null;
+        this.submitUrl = null;
     }
 
     /**
@@ -101,6 +103,7 @@ export default class AddThreepid {
             phoneCountry, phoneNumber, this.clientSecret, 1,
         ).then((res) => {
             this.sessionId = res.sid;
+            this.submitUrl = res.submit_url;
             return res;
         }, function(err) {
             if (err.errcode === 'M_THREEPID_IN_USE') {
@@ -198,12 +201,23 @@ export default class AddThreepid {
     async haveMsisdnToken(msisdnToken) {
         const authClient = new IdentityAuthClient();
         const identityAccessToken = await authClient.getAccessToken();
-        const result = await MatrixClientPeg.get().submitMsisdnToken(
+
+        let result;
+        if (this.submitUrl) {
+            result = await MatrixClientPeg.get().submitMsisdnTokenOtherUrl(
+                this.submitUrl,
+                this.sessionId,
+                this.clientSecret,
+                msisdnToken,
+            );
+        } else {
+            result = await MatrixClientPeg.get().submitMsisdnToken(
                 this.sessionId,
                 this.clientSecret,
                 msisdnToken,
                 identityAccessToken,
             );
+        }
         if (result.errcode) {
             throw result;
         }