Update new user invitation email with an avatar

What does this MR do?

• This creates an experiment, using gitlab-experiment for a new invite email design that includes the inviter's avatar image for new members only.
• Allows for namespaced experiments

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

Failures in CI?

Until #300104 (comment 494730619) is resolved, this may fail CI...so we'll wait to merge until then.

How to test?

• In Rails console(bundle exec rails console), issue this command to put yourself in the experiment

Feature.enable_percentage_of_time(:members_invite_email, 100)

• simply run the same command with 0 instead of 100 to see the before. May need to send a few invites before the cache clears and updates sidekiq to see the change.
• Go to a group or project of your choice
• Go to "members"
• Invite a new member via mail
• Go to http://localhost:3000/rails/letter_opener/

Screenshots (strongly suggested)

Before	After

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

Related to #296966