Remove Admin > Settings > Templates link from side bar having an invalid license

Description

Templates is an EE feature. Docs: https://docs.gitlab.com/ee/user/admin_area/settings/instance_template_repository.html

It's guarded by License.feature_available?(:custom_file_templates) and only available on Premium, so if you don't have the EE license or it's not the right tier, you will see a blank page.

We should remove the link from the side left bar if the user doesn't have the right license.