Layout Slider: External URL Field Overlaps at Some Breakpoints

## Summary
In the Layout Slider Background settings, the External URL field overlaps the surrounding fields at some widths. Check the field's responsive breakpoint.

## Screenshot
`External URL Breakpoint.png`
![External URL Breakpoint](https://gist.githubusercontent.com/Misplon/d53a7406dbce8601a252ca58a9d4ef9d/raw/external-url-breakpoint.png)