Early access program participation checkbox

What does this MR do and why?

Early access program participation checkbox

It provides ability to join GitLab Early access program through group settings.

Issue

Screenshots or screen recordings

Before	After

The link to GitLab Early Access Program goes here: https://about.gitlab.com/community/early-access/

Why is it different from initial UX?

We had initial UX proposal for this one. There are few reasons why is implementation different:

• We wanted to highlight program name (Early Access Program)
• We received confirmation from legal that if we have exclusive marketing channel (with this program only related emails) then we can soft opt-in users without notice that they agree on marketing communication.
• Disabled checkbox state when main ("Use experimental and beta features") is unchecked doesn't work here yet because it will require more efforts from frontend perspective. It is the same reason why it doesn't work for "Security policy management" and "Duo features" sections above.

How to set up and validate locally

1. Enable feature flag Feature.enable(:early_access_program_toggle)
2. Go to enterprise edition licensed group settings
3. Apply to Use experiment and beta features and Participate in the GitLab Early access program in (Permissions and group features)
4. Through rails console check if your group has correct namespace settings attributes: group.namespace_settings.attributes.slice('early_access_program_participant', 'early_access_program_joined_by_id')