installation: re-enable WSGI pre-loading
- Re-enables WSGIImportScript directive in generated Apache VirtualHost snippet in order to pre-load Invenio WSGI application processes for faster site replies. Generated snippets are fit for mod_wsgi 3.x. Sites using old legacy mod_wsgi 2.x versions need to comment out or move the WSGIImportScript directive; see the INSTALL file for more information.