[gateway] improve "switch account" behaviour

[leleueri]

💥 Describe the bug

When ID first login is enable and when the idfirst login page is accessed using the login_hint parameter, if an automatique redirection is done in a custom template using this JS:

<script th:if= "${param.login_hint != null && param.login_hint != ''}">
    $(document).ready(function(){
        $('#form').submit();
    });
</script>

Then the switch account link doesn't work in the login page because it includes the login_hint parameter.

🌄 To Reproduce

Steps to reproduce the behaviour:

1. create a custom identifier first login page with the JS snippet above to validate automatically the form if login_hint is present
2. call the authorization endpoint with a login_hint parameter
3. the login page is displayed
4. click on "switch account"
5. the identifier first login page is displayed but as the login_hint parmeter is present, the form is submitted automatically

🌈 Expected behaviour

the identifier first login page is displayed without login_hint

Current behaviour

the identifier first login page is called with login_hint so the form is automatically validated

[patelami]

Env: 3.15.x

Result: PASS

Given I have enabled ID first login
And I initiate the login flow
Then i should see username with the value in username field readonly mode
Then I click on switch account
And I add login_hint=a2 in parameter in URL
Then I should see a2 added to the login form in username field

switch_account_behaviour.mov