Hide OmniAuth buttons in signup page

What does this MR do and why?

Show specific OmniAuth buttons in signup page

If the omniauth option allow_single_sign_on is false hide the OmniAuth buttons in signup page. That's because in this scenario an account must be created previously.

If allow_single_sign_on is an array, show only those providers in the signup page.

See https://docs.gitlab.com/ee/integration/omniauth.html#configure-common-settings

Relates to #415478 (closed)

Note: there is a setting that allows to signup, that's different from allow_single_sign_on.

Changelog: changed

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Screenshots or screen recordings

When allow_single_sign_on: false

Before	After

When allow_single_sign_on: ['saml']

Before	After

How to set up and validate locally

1. On gitlab.yml, set up allow_single_sign_on to false or ['github'] and restart the instance.

  omniauth:
    allow_single_sign_on: false # It means that signing in using your OmniAuth provider account is only allowed if the user has a pre-existing account in the current instance.

2. Go to /users/sign_up check that there are no buttons.