Draft: Introduce feature flag and UI for simulate pipeline feature

What does this MR do and why?

For #337282 (closed)

This adds the simulate_pipeline feature flag and replaces the Lint tab of the pipeline editor with the Validate tab. The UI is static for now. In future MRs, we will add the simulate pipeline functionality to it.

NOTE: Decided to break this down into !89497 (merged) and !90345 (merged) for easier frontend review

Feature Breakdown

frontend	MR
Introduce feature flag to replace Lint tab with Validate tab in the pipeline editor	!89497 (merged)
Add popover and NEW badge to the Validate tab	!90345 (merged)
Add simulation functionality to the Validate tab	TODO
Roll out feature flag	#364257 (closed)

Screenshots or screen recordings

Before (Lint Tab)	After (Validate Tab)

NEW badge disappears when exiting from the tab:

Screen_Recording_2022-06-06_at_23.49.40

How to set up and validate locally

1. Enable the simulate_pipeline feature flag (rails c > Feature.enable(:simulate_pipeline))
2. Go to the pipeline editor (CI/CD > Editor)
3. The Lint tab should be replaced with the Validate tab.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.