Add social sign up options to trial registration page

What does this MR do?

Experiment with adding social sign in options to trial registration page

Related to #262845 (closed)

Testing

FF to enable on your local rails console

Feature.enable_percentage_of_time(:trial_registration_with_social_signin_experiment_percentage, 100)

• Add github after google_oauth2 to development: section in the config/gitlab.yml file and gdk restart
• To test with Google follow https://developers.google.com/identity/sign-in/web/sign-in

development:
  <<: *base
  omniauth:
    allow_single_sign_on: ["google_oauth2"]
    block_auto_created_users: false
    providers:
    - { name: 'google_oauth2',
        app_id: '',
        app_secret: '',
        args: { access_type: 'offline', approval_prompt: 'Heyyah' } }
    - { name: 'github',
        app_id: 'YOUR_APP_ID',
        app_secret: 'YOUR_APP_SECRET',
        url: "https://github.com/",
        verify_ssl: false,
        args: { scope: 'user:email' } }

http://localhost:3000/-/trial_registrations/new

Screenshots

Before	After

Also we didn't break the main sign up view

Recording

 13 |             5 |     220 |          1 | 2020-12-04 22:49:15.454916+03 | 2020-12-04 22:49:15.454916+03 |              | {"github_signon": false, "google_signon": true}

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team