Replaces "Libre" with "Core"

• For #2025 (closed)
• Search and replace all www, except:
  • /public/*
  • .png and .jpg
  • "libre" mentioned in urls
  • PM handbook
  • resellers.yml

Review Apps:

• https://replace-libre-w-core.about.gitlab.com/features/
• https://replace-libre-w-core.about.gitlab.com/2018/03/22/gitlab-10-6-released/
• https://replace-libre-w-core.about.gitlab.com/pricing/
• https://replace-libre-w-core.about.gitlab.com/pricing/self-hosted/feature-comparison/