Remove $page-vertical-gap SASS variable which we never use

Problem/Motivation

The various project shows that we never overridden $page-vertical-gap SASS variable and that assumption that this variable should control both header and content bottom margin is probably overly optimistic. Also, we can not by default assume any kind of bottom margin for region_content because of supports of paragraph containers with bg styles - in this case, you do not want some kind of a vertical gap between content and footer.

Proposed resolution

- Remove $page-vertical-gap SASS variable.
- Remove page__header, page__main and region-content elements bottom margin.
- Introduce helper $container-spacer-y and use this value as a default for $region-content-footer-margin. We do this because we need a general variable for this and in the future, we will probably expand usage of this variable.