Fix double escaping of slogan

[MorrisJobke]

Slogan is already escaped in https://github.com/nextcloud/server/blob/cce4c285dbfd6957f50112b234b3545ebcceac54/apps/theming/lib/ThemingDefaults.php#L142

Fixes nextcloud/server#7460

[blizzz]

Not convinced. getSlogan can be overridden by themes, for instance, we cannot be sure it always arrives sanitized. I'd rather remove the sanitation in ThemingDefaults. Also, apparently there is also a double escaping in the Login Controller as of now:

core/Controller/LoginController.php
193:            Util::addHeader('meta', ['property' => 'og:description', 'content' => Util::sanitizeHTML($this->defaults->getSlogan())]);

Requires more changes however, but I feel more comfortable when it is sanitized where the output takes place.