Fix panel "more actions" button layout

What does this MR do?

Fixes the layout of the More actions button which appears in dashboard panels.

This MR was opened as a follow-up for this: !39138 (comment 395108860)

This is the implementation MR of this issue: #235997 (closed)

Screenshots

Before	After
Incorrect width, off-center icon	Correct width, centered icon

Video (for "After" state)

Screen_Recording_2020-08-17_at_22.01.02

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

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• 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