Refactor in-product-marketing messages

What does this MR do?

This refactors the in-product-marketing emails into separate classes. So far everything was in a helper, by separating it into classes, we:

• Do not need to pass the state (series, track, group) to each method separately
• Can easily extend this for new emails (issue gitlab-org/growth/engineering#5437 (closed))
• Can easily change content that requires state (issue: gitlab-org/growth/team-tasks#375)

This MR looks huge, but is mostly moving code around.

How to test the email locally if you're interested to see how it looks like

1. bin/rails c
2. NotificationService.new.in_product_marketing(User.first.id, Group.first.id, :create, 0)
3. Go to http://localhost:3000/rails/letter_opener/

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because it is a developer-facing change.
• 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