feat(GlLink): Add safe-link directive to make the links secure

What does this MR do?

This MR aims to add safe-link directive (!1457 (merged)) to GlLink component.

Related #823 (closed)

Follow-up tasks

These would be tackled in a different MR.

1. Create an MR to add directive to GlButton component
2. Deprecate or Remove re_mixin.js and related codebase

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.