fix: align completion modal buttons (#39509)

client/src/components/Header/components/Login.js

@@ -29,7 +29,7 @@ function Login(props) {
   return (
     <Button
       bsStyle='default'
-      className={(block ? 'btn-cta-big' : '') + ' signup-btn btn-cta'}
+      className={(block ? 'btn-cta-big btn-block' : '') + ' signup-btn btn-cta'}
       data-test-label={dataTestLabel}
       href={href}
       onClick={() => gtagReportConversion()}

client/src/templates/Challenges/components/CompletionModal.js

@@ -217,14 +217,7 @@ export class CompletionModalInner extends Component {
         </Modal.Body>
         <Modal.Footer>
           {isSignedIn ? null : (
-            <Login
-              block={true}
-              bsSize='lg'
-              bsStyle='primary'
-              className='btn-cta'
-            >
-              Sign in to save your progress
-            </Login>
+            <Login block={true}>Sign in to save your progress</Login>
           )}
           <Button
             block={true}