Group SAML settings link checks license

We allow group owners to configure SAML on GitLab.com but only if the group has a license for the feature.

This MR ensures that the link to that settings page is only shown when the feature is available for that group.

See: https://gitlab.slack.com/archives/C8B6WPDH7/p1528993091000090?thread_ts=1528934502.000339&cid=C8B6WPDH7 (internal slack link)

Acceptance criteria

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the style guides
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

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