Verify vue3 migration in github importer flow

What does this MR do and why?

Adds vue 3 migration Feature flag for github importer, starting from after you click "Authenticate" on the page /import/github/new

Fixes any problems observed. In this case there was just a console warning regarding a required gl-modal prop

I noticed pages/import/github/new/index.js still referred to a feature flag that no longer exists (!233393 (merged)) so I removed that reference and created a followup for further cleanup #624301.

References

#619528

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In Vue 3 mode, turn on the feature flag
2. Navigate to projects/new#import_project and choose Github
   1. Note, the GitHub login page is just haml
3. You'd need a personal access token to your GitHub account. Click Authenticate

MR acceptance checklist

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