Fix design view overlay positioning

What does this MR do?

This fixes positioning of design overlay pins when viewing design.

Specifically, the div wrapping gl-alert was causing the <image> element to be pushed down the page, thus causing the overlay to appear "above" where it was meant to be.

Current problem (overlay isn't directly over the image)	Expected (overlay element is positioned over the image)

This MR pulls GLAlert out of the div that wraps design-image and design-overlay so as to not interfere with any positioning

Screenshots

No error	With error (notice that pins are in correct position)

Risks

Pins created since GlAlert was added (2019-12-20) will be positioned 16px lower than what the user intended. Repositioning of pins is currently scheduled for %"12.8" and so if it's critical that pins are positioned correctly, the user will be able to reposition them once this ships.

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

Testing

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