Update Libre to Core

Executive Summary

Libre

• The feedback from our Community Team (@matteeyah) and Engineering team (@balasankarc @northrup) is that "Libre" is a confusing name for a pricing tier
• Libre is an uncommon English word so it is difficult for non-native English speakers to understand
• Libre is not a meaningful term to folks unfamiliar with open source software
• Libre is a term that evokes strong emotion for those who are inside the open source community. In particular, referring to EE (which has proprietary code even in the non-paid version) as "Libre" can be controversial or confusing since it typically refers to completely open source software.

Core

• The consensus is that "Core" is a more appropriate tier name. [1] [2] [3]
• Core is easy for non-native English speakers to understand
• Core is easy for folks unfamiliar with open source software to understand
• Core doesn't cause controversy within the open source community.
• Core ties into our "open core" messaging. If a feature is part of the Core tier, then it is an open source feature that is part of our open core.

Full Details

Full discussion can be found in https://gitlab.com/gitlab-com/marketing/general/issues/2214 and https://gitlab.slack.com/archives/C0AKZRSQ5/p1519330809000344

Next Steps

• If this MR is approved @williamchia will create additional MRs to update Libre to Core universally.