Moved themes from /themes/vufind/ to just /themes/ -- this simplifies file...

Moved themes from /themes/vufind/ to just /themes/ -- this simplifies file paths and fixes a bug in install.php that generated an invalid Apache configuration.  This increases the risk of conflict with themes from a different module, but using VuFind in combination with other theme-using modules is going to require extra customization anyway; not worth complicating the common use case.