Migrate gitlab import modal to GlModal

What does this MR do and why?

Replaces the haml GitLab import modal with a GlModal.

• Updates the bitbucket modal specs to be shared examples
• Adds additional tests for the gitlab import modal

Screenshots or screen recordings

Before	After

How to set up and validate locally

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

1. Navigate to the new projects form http://<gdk>/projects/new
2. Click the "GitLab.com" button

The new modal should appear

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 #344558 (closed)