Create the XWiki Days application

On https://dev.xwiki.org/xwiki/bin/view/Community/XWikiDays/, there's a long list with XWiki days entries written manually over time. In order to ease the XWiki days management, an application with a livetable will be needed. The new livetable should replace the existing list and a migrator from the existing XWiki days blog posts to the new structure should be provided.
There's already created a beta version of a structure with the following pages:
Community.XWikiDays.XWikiDayClass
Community.XWikiDays.XWikiDaySheet
Community.XWikiDays.XWikiDayTemplate
Community.XWikiDays.Translations

We should either continue on the existing structure, which is really basic, either create the app from scratch.