Package list to graphql: add feature flag

What does this MR do and why?

This MR is part of the work to refactor the Package list app from REST to GraphQL/Apollo

In this MR we are:

• adding a new feature flag to proceed with the refactor
• loading the new app when the feature flag is on
• fixing the test to only run on old code
• Clean up and remove vuex reference from main app

We will likely need 2 or 3 more MRs to complete the work.

Feature flag rollout issue: #341136 (closed)

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. in rails console Feature.enable(:package_list_apollo)
2. navigate to the package list

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.

Related to #330846 (closed)