Remove XWiki Enterprise and transform it into a Knowledge Base Flavor

Goal is to:

• Distribute the platform distibution (instead of the XE distribution). This distribution allows to choose the flavor to use on first start
• Remove XWiki Enterprise
• Introduce a KB Flavor in xwiki-platform which will have the same features as XE has now (in the future we may want to remove the Blog application as has been discussed previously and more generally tune the flavor to be focused on the KB domain).