Make it easier for people to share their enthusiasm for GitLab

People want to share their love of GitLab. It shouldn't be so hard.

• Add Opengraph meta data to templates with sensible defaults #1366 (closed).
• Make meta data configurable on templates #517
• Add social share buttons on blog posts.
• In the onboarding experience, provide links to make it easy to share (e.g., tweet intent, linkedin, FB share, etc)

Remember to rethink how the default install handles itself, too: gitlab-org/gitlab-ce#10858 :)

@kousu Thanks, Nick! Good point. It's all connected.