Draft: PoC: Add current trial status for namespace to sidebar

What does this MR do?

Initiates a new experiment for a trial upgrade nudge. The nudge itself takes the form of a new sidebar item which shows up for owners of the namespace being trialed. The sidebar item conveys how much time is left in the trial, and clicking on it takes the user to the billing plans selection page.

Screenshots

• Add screenshots

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

Addresses to #288014 (closed)

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Engineering Work Plan

• Create feature flag (yml-version, I think) for the experiment
• Add it to EXPERIMENTS yml as well?
• Figure out how to record the experiment subject & variant (e.g. can we pass a variant override to record_experiment_user?)
• Look into the view – will we be using Vue for it?
  • If so, what components already exist for what we want to make?
• Add the sidebar component
• Figure out the “pre-segmenting” of trial namespace owners who are not members of any other paid namespaces
• Figure out how to have a click/tap of the sidebar component take them to the billing plans page
• Wrap it up with a pretty little 🎀