Scrollable design view

What does this MR do?

Modifies the behaviour of the Design view such that design images:

• default to max-width: 100%;
• are allowed to overflow (vertically)

Why?

Firstly, design images whose height far outweighs it's width are hard to read properly due to the fact that the entire image must be displayed in the viewport. (for example, see this design and this discussion in slack for context/further discussion).

Secondly, in preparation for #13217 (closed) (!22863 (merged)), this change is necessary because of the desired zooming behaviour.

Screenshots

Before	After (enabling user to scroll vertically)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers