feat: improve meta tag fallback handling to provide more meaningful values when not overriden manually

I've run across all important district and general page types and improved get_meta_image and get_meta_description behaviour to provide more meaningful values if user doesn't specify custom search_image and search_description by hand.

I also removed limitting description length to 150 chars in case perex gets used as the desc source. In this case, it is very likely we want to have it whole in the desc.