Fix user migration for context on trial redesign experiment

What does this MR do and why?

Fix user migration for context on trial redesign experiment

• correctly allow cookie migration to actor after logging in https://gitlab.com/gitlab-org/ruby/gems/gitlab-experiment#cookies-and-the-actor-keyword
• add missing testing for experiment in feature specs
• fix cookie migration with use of actor keyword and triggering in welcome controller
• fix sso variable needed for the experiment for trial: true in the view layer
• see thread for a lot of the explanation and learnings here: !135635 (comment 1629431559)

MR acceptance checklist

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Related to #425215