refs #5222 Remove Zeitgeist theme from Piwik and support only Morpheus

Removed Zeitgeist. Morpheus is the new default theme allowing theme designers to easily overwrite this theme. This is quite easy now. Also fixes some issues for instance the Zeitgeist buttons were still used in Installation and CoreUpdater. In All Websites Dashboard were still some icons (eg link, search, ...) from Zeitgeist used. Bye bye Zeitgeist! You did a nice job over the past years