Merge pull request #12988 from owncloud/logfile_download

lib/private/log/owncloud.php

lib/public/appframework/http/datadownloadresponse.php

settings/admin.php

settings/ajax/getlog.php

settings/ajax/setloglevel.php

settings/application.php

settings/controller/logsettingscontroller.php

settings/js/admin.js

settings/js/log.js

settings/routes.php

settings/templates/admin.php

tests/settings/controller/logsettingscontrollertest.php