Add invite email experiment

What does this MR do?

This creates an experiment for a new invite email design that is simpler for our users and only has one call to action instead of multiple. In the first iteration, this is only for new users and we're tracking how many more users accept the invitation with the new design compared to the old one.

We'll roll out the experiment with a feature flag invite_email_experiment for X% to see if it improves acceptance rate to groups.

Issue: https://gitlab.com/gitlab-org/growth/team-tasks/-/issues/182

How can I test this locally?

Activatae the feature flag in your rails console (bin/rails c)

  Feature.enable(:invite_email_experiment)

Send email

1. Go to http://localhost:3000/groups/h5bp/-/group_members
2. Add any member by email
3. Go to http://localhost:3000/rails/letter_opener/ and refresh

Screenshots

Current	Experiment

Snowplow Events

1. When sending an email:

   {
     name: Growth::Acquisition::Experiment::InviteEmail,
     action: 'sent',
     property: 'control_group' OR 'experiment_group'
   }

2. When user opens the invite link, we also send a hash that is consistent for each invite. We need to unify on this hash, because the link can be opened multiple times

   {
     name: Growth::Acquisition::Experiment::InviteEmail,
     action: 'opened',
     property: 'control_group' OR 'experiment_group',
     value: UNIQUE_HASH_PER_INVITE
   }

3. When user accepts the invitation

   {
     name: Growth::Acquisition::Experiment::InviteEmail,
     action: 'accepted',
     property: 'control_group' OR 'experiment_group',
     value: UNIQUE_HASH_PER_INVITE
   }

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