Pipeline Authoring - CI Variables improvements
## Overview
This MVC epic represents a collection of issues related to improvements to CI variables.
## Why is this important to the business?
* Our backlog now contains almost 500 [issues](https://gitlab.com/gitlab-org/gitlab/-/issues/?sort=popularity&state=opened&label_name%5B%5D=group%3A%3Apipeline%20authoring&label_name%5B%5D=CI%20variables&first_page_size=20) that are labeled as CI variables which is \~20% of the entire pipeline authoring backlog, many of those issues are highly required and top voted (FYI last year the number was around 300, which means there is a growing demand from our users and customers)
* Variables were mentioned in one of our recent [blogs](https://about.gitlab.com/blog/2021/02/22/continuously-improving-ci-lovability/) as one key path to loveability.
* Variables is a [very buggy area for our users](https://gitlab.com/groups/gitlab-org/-/epics/5124 "Bugs: Variables")
* On [our UX research](https://gitlab.com/groups/gitlab-org/-/epics/5901 "UX Scorecard: Control scope for CI/CD variables") users are not provided enough control over variables to use them optimally.
* Variables were recognized as one of the biggest blockers for [Jenkins migration](https://gitlab.com/gitlab-org/gitlab/-/issues/412659 'Set a predefined list of values DYNAMIC from a FILE/$OPTION/COMMAND for CI environment variables and surface them in the "Run pipeline" page form')
## Analysis to support this prioritization
* The running theme around the issues attached to this epic is "confusion", "hard to understand", "performance" and "getting out of hand". Because variables are one key path to lovability, these technical debt areas have to be addressed if we're ever going to get to that maturity level.
epic