Avoid redundant URL encoding (#4555)

* Do not double encode fallback redirect URL

Signed-off-by: Marcel Fabian Krüger <zauguin@gmail.com>

changelog.d/4555.bugfix

+Avoid redundant URL encoding of redirect URL for SSO login in the fallback login page. Fixes a regression introduced in [#4220](https://github.com/matrix-org/synapse/pull/4220). Contributed by Marcel Fabian Krüger ("[zaugin](https://github.com/zauguin)").

synapse/static/client/login/js/login.js

@@ -49,7 +49,7 @@ var show_login = function() {
     $("#loading").hide();
 
     var this_page = window.location.origin + window.location.pathname;
-    $("#sso_redirect_url").val(encodeURIComponent(this_page));
+    $("#sso_redirect_url").val(this_page);
 
     if (matrixLogin.serverAcceptsPassword) {
         $("#password_flow").show();