Design view: Do not restrict design image width

What does this MR do?

Raised in #201780 (closed). This MR removes some CSS classes so that design images will fill the available space (to a max width of 100%) on initial load.

Previously we were, by default, restricting image height/width to 100%. This meant that long-and-slim images would be very hard to see, and the user could not zoom in to inspect closely.

Screenshots

Before	After
Image with large dimensions (much larger than viewport, > 2000px)	-
Reveal	Reveal
Image with small dimensions (width is around 400px). Notice that image is allowed to overflow on initial load now	-
Reveal	Reveal

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team