Enable iframe-based silent refresh in Code Flow example

[jeroenheijmans]

In #24 we're trying to get Code + PKCE flow working. However, I'm very keen on getting iframe-based silent refresh to work before making Code Flow the new master branch example (as per the newest recommendations).

The reason that iframe-based silent refresh is so important in my opinion, is this scenario:

1. User lands in your application, logged in on the Identity Server, but not yet in your app
2. The app fires up, has no refresh token (in fact storage is empty for oauth lib)
3. The app first tries the iframe based silent refresh, as it should work because the user has a session at the IDS
4. The silent refresh should work, and the user gets logged in without a prompt

This should've been fixed in manfredsteyer/angular-oauth2-oidc#600 with version 9.1.0, so let's try that and merge it into the #24 branch (and later into the master branch).

[jeroenheijmans]

Just logged manfredsteyer/angular-oauth2-oidc#777 because I think it's not working yet.

[jeroenheijmans]

Workaround added in #35