fix(GlDropdown): Add styling for search within gldropdown

What does this MR do?

Problem

Currently, gl-search-box-by-type isn't being styled within a dropdown if someone were to use one in a dropdown. In the new-dropdown-with-search example, we are hard-coding an m-2 class, whereas we aren't adding this class to the search box in the with header story. This is a perfect example of the discrepancy on consistent usage of a search box within a dropdown.

Also, the current structure in the with header story is:

<gl-dropdown-text><gl-search-box-by-type /></gl-dropdown-text>

This seems to be a temporary measure to prevent the hover styles coming through when wrapping in a gl-dropdown-item wrapper (which shows a light gray background). Either way, the search box ends up getting strange styling (see corresponding issue).

Solution

This MR removes the hard-coded m-2 class on the with search example and adds CSS to target the gl-search-box-by-type so styling is out-of-the-box and no hard-coded styling is necessary for the person implementing the component. It can now be a standalone component within the dropdown, next to it's <gl-dropdown-item> and <gl-dropdown-text> brothers and sisters.

Screenshots

Before	After

Integration MR

Does this MR meet the acceptance criteria?

Conformity

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• GitLab UI's contributing guidlines.
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• Added the ~"component:*" label(s) if applicable.

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
• Security reports checked/validated by a reviewer from the AppSec team

Closes #976 (closed)