apply best practices to subscribe widget
apply best practices to subscribe widget

- use before/after variables instead of hardcoded HTML
- apply `widget_title` filter

close #624

noplanman/podlove-publisher,katrinleinweber/podlove-publisher,podlove/podlove-publisher,katrinleinweber/podlove-publisher,podlove/podlove-publisher,podlove/podlove-publisher,katrinleinweber/podlove-publisher,podlove/podlove-publisher,noplanman/podlove-publisher,noplanman/podlove-publisher,katrinleinweber/podlove-publisher,noplanman/podlove-publisher,podlove/podlove-publisher,noplanman/podlove-publisher,podlove/podlove-publisher,katrinleinweber/podlove-publisher