Make media-slide overlay responsive settings mandatory and provide default values

Problem/Motivation

I guess we had planned to inherit settings from small to larger devices, but this is way to complicate.

Steps to reproduce

Proposed resolution

Make the following fields mandatory:

• field_overlay_position => already is mandatory, the default value is also already correct
• field_overlay_sizing => change the default value to: 'full-size'
• field_overlay_position_md => default value: 'disabled'
• field_overlay_sizing_md => default value: 'seperate-box'
• field_overlay_position_lg => default value: 'disabled'
• field_overlay_sizing_lg => default value: 'seperate-box'

Remaining tasks

User interface changes

API changes

Data model changes