Move PleineLune to Marketplace #4285

• Remove PleineLune from Core
• Create repository for PleineLune
• Add Piwik Team to repository
• Enable git diff mails and add it to git.piwik.org
• Add repository as a submodule in piwik
• Update PleineLune repository to have readme, plugin.json and screenshots
• Publish PleineLune in Marketplace
• Remove AnotherBlackTheme from Marketplace

To make sure we have at least one theme in the Marketplace