Resolve "Replace fa-github icon with GitLab SVG GitHub icons"

What does this MR do?

• Replaces Font Awesome icons with GitLab SVGs.
• Updates 2 buttons to use the new gl-button styles while touching one of the haml files (one of the buttons contains an updated icon).

Screenshots

Location	Before	After
Projects > New “CI/CD for external repo” tab
Projects > Import > GitHub (requires GitHub personal access token)
Projects > New “Import project” tab
Projects > New “Import project” tab (These ones are coming along for the ride since the file was open)
Projects > Import > GitHub (I had to remove a condition in gitlab/app/views/import/github/new.html.haml to get the UI to appear)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

N/A

Security

N/A

Closes #246512 (closed)