Switch JS code to url-safe base64-encoding

ui/src/sso.js

@@ -13,7 +13,7 @@ function bufferEncode(value) {
 }
 
 function bufferDecode(value) {
-    return Uint8Array.from(atob(value), c => c.charCodeAt(0));
+    return Uint8Array.from(atob(value.replace(/-/g, '+').replace(/_/g, '/')), c => c.charCodeAt(0));
 }
 
 function verifyAssertion(url, assertedCredential) {