Change unsubscribe language for email campaign

What does this MR do?

This changes the text to unsubscribe from in-product marketing emails on self managed, moves up the unsubscribe link in the email and adds a link to the general marketing preferences.

Issue: gitlab-org/growth/product#1681 (closed) (note that the issue states it is only about "CREATE Emails", but the header/unsubscribe text changes need to affect all emails.

How to test locally

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)

before	after

Does this MR meet the acceptance criteria?

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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