Improved footer


@MTRichards and @jancborchardt please have a look.

Like discussed in #3791 I moved the construction of the footer to defaults.php which allows us to change the footer in one central place.

During this changes I discovered that for the enterprise theme we don't show the version at all which is probably bad because this way it is not possible to find out which ownCloud version is deployed. So I moved this out of the footer for the personal and admin settings. Please have a look if this works for the enterprise edition.