Implement password reset

This adds a link to the login screen with "Forgot your password?". Clicking it
takes you to a form with fields for an email address and a new password. This
makes the same API calls as the Angular SDK.

Manually tested resetting + not clicking link + invalid email and it all seems
to work.

src/PasswordReset.js

@@ -0,0 +1,104 @@
+/*
+Copyright 2015, 2016 OpenMarket Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+var Matrix = require("matrix-js-sdk");
+
+/**
+ * Allows a user to reset their password on a homeserver.
+ *
+ * This involves getting an email token from the identity server to "prove" that
+ * the client owns the given email address, which is then passed to the password
+ * API on the homeserver in question with the new password.
+ */
+class PasswordReset {
+
+    /**
+     * Configure the endpoints for password resetting.
+     * @param {string} homeserverUrl The URL to the HS which has the account to reset.
+     * @param {string} identityUrl The URL to the IS which has linked the email -> mxid mapping.
+     */
+    constructor(homeserverUrl, identityUrl) {
+        this.client = Matrix.createClient({
+            baseUrl: homeserverUrl,
+            idBaseUrl: identityUrl
+        });
+        this.clientSecret = generateClientSecret();
+        this.identityServerDomain = identityUrl.split("://")[1];
+    }
+
+    /**
+     * Attempt to reset the user's password. This will trigger a side-effect of
+     * sending an email to the provided email address.
+     * @param {string} emailAddress The email address
+     * @param {string} newPassword The new password for the account.
+     * @return {Promise} Resolves when the email has been sent. Then call checkEmailLinkClicked().
+     */
+    resetPassword(emailAddress, newPassword) {
+        this.password = newPassword;
+        return this.client.requestEmailToken(emailAddress, this.clientSecret, 1).then((res) => {
+            this.sessionId = res.sid;
+            return res;
+        }, function(err) {
+            if (err.httpStatus) {
+                err.message = err.message + ` (Status ${err.httpStatus})`;
+            }
+            throw err;
+        });
+    }
+
+    /**
+     * Checks if the email link has been clicked by attempting to change the password
+     * for the mxid linked to the email.
+     * @return {Promise} Resolves if the password was reset. Rejects with an object
+     * with a "message" property which contains a human-readable message detailing why
+     * the reset failed, e.g. "There is no mapped matrix user ID for the given email address".
+     */
+    checkEmailLinkClicked() {
+        return this.client.setPassword({
+            type: "m.login.email.identity",
+            threepid_creds: {
+                sid: this.sessionId,
+                client_secret: this.clientSecret,
+                id_server: this.identityServerDomain
+            }
+        }, this.password).catch(function(err) {
+            if (err.httpStatus === 401) {
+                err.message = "Failed to verify email address: make sure you clicked the link in the email";
+            }
+            else if (err.httpStatus === 404) {
+                err.message = "Your email address does not appear to be associated with a Matrix ID on this Homeserver.";
+            }
+            else if (err.httpStatus) {
+                err.message += ` (Status ${err.httpStatus})`;
+            }
+            throw err;
+        });
+    }
+}
+
+// from Angular SDK
+function generateClientSecret() {
+    var ret = "";
+    var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
+
+    for (var i = 0; i < 32; i++) {
+        ret += chars.charAt(Math.floor(Math.random() * chars.length));
+    }
+
+    return ret;
+}
+
+module.exports = PasswordReset;