Deprecate `sso_immediate_redirect` and add welcome-page only option (#19437)

* Deprecate `sso_immediate_redirect` and add welcome-page only option

The old `sso_immediate_redirect` option is kept in code for backwards compatibility, but is replaced by `sso_redirect_options.immediate`, where a new `on_welcome_page` option is also introduced to only target entry points to the app. 

The path matching is a bit brittle, but still ideal in the sense of not showing UI while the app realizes it needs to redirect.

The new welcome page-only option is fully intended to avoid breaking permalinks and such while still redirecting when needed. In future, other options might be added here to further tune the experience.

* Add note about guests

docs/config.md

@@ -22,9 +22,19 @@ For a good example, see https://develop.element.io/config.json.
      `default_hs_url` is specified. When multiple sources are specified, it is unclear
      which should take priority and therefore the application cannot continue.
    * As of Element 1.4.0, identity servers are optional. See [Identity servers](#identity-servers) below.
-1. `sso_immediate_redirect`: When `true`, Element will assume the default server supports SSO
+1. `sso_redirect_options`: Optionally defines how Element will behave with a server which supports
-   and attempt to send the user there to continue (if they aren't already logged in). Default
+   Single Sign On (SSO). By default, Element will do nothing special and simply show a button where
-   `false`. Note that this disables all usage of the welcome page.
+   needed for the user to click to navigate to the SSO system. This behaviour can be tuned with the
+   config options below (as properties of the `sso_redirect_options` object). None of the options apply
+   if Element thinks the user is already logged in, and similarly Element will assume the default server
+   supports SSO if these redirect options are used.
+   * `immediate`: When `true` (default `false`), Element will automatically redirect all unauthenticated
+     users to the SSO system to log in regardless of how they reached the app. This overrides the use of
+     other redirect options.
+   * `on_welcome_page`: When `true` (default `false`), Element will automatically redirect all unauthenticated
+     users to the SSO to log in if the user lands on the welcome page or no specific page. For example,
+     https://app.element.io/#/welcome and https://app.element.io would redirect if set up to use this option.
+     This can be useful to maintain guest experience until an account is needed.
 1. `features`: Lookup of optional features that may be force-enabled (`true`) or force-disabled (`false`).
    When features are not listed here, their defaults will be used, and users can turn them on/off if `showLabsSettings`
    allows them to. The available optional experimental features vary from release to release and are

src/vector/app.tsx

@@ -30,7 +30,7 @@ import AutoDiscoveryUtils from 'matrix-react-sdk/src/utils/AutoDiscoveryUtils';
 import { AutoDiscovery } from "matrix-js-sdk/src/autodiscovery";
 import * as Lifecycle from "matrix-react-sdk/src/Lifecycle";
 import type MatrixChatType from "matrix-react-sdk/src/components/structures/MatrixChat";
-import SdkConfig from "matrix-react-sdk/src/SdkConfig";
+import SdkConfig, { parseSsoRedirectOptions } from "matrix-react-sdk/src/SdkConfig";
 import { logger } from "matrix-js-sdk/src/logger";
 
 import { parseQs, parseQsFromFragment } from './url_utils';
@@ -159,7 +159,13 @@ export async function loadApp(fragParams: {}) {
     const [userId] = await Lifecycle.getStoredSessionOwner();
     const hasPossibleToken = !!userId;
     const isReturningFromSso = !!params.loginToken;
-    const autoRedirect = config['sso_immediate_redirect'] === true;
+    const ssoRedirects = parseSsoRedirectOptions(config);
+    let autoRedirect = ssoRedirects.immediate === true;
+    // XXX: This path matching is a bit brittle, but better to do it early instead of in the app code.
+    const isWelcomeOrLanding = window.location.hash === '#/welcome' || window.location.hash === '#';
+    if (!autoRedirect && ssoRedirects.on_welcome_page && isWelcomeOrLanding) {
+        autoRedirect = true;
+    }
     if (!hasPossibleToken && !isReturningFromSso && autoRedirect) {
         logger.log("Bypassing app load to redirect to SSO");
         const tempCli = createClient({