Update frequent_items to upcoming resp styling

• Requirements (issue)
• Tasks (epic)
• Full Context Spike MR (permanent draft)
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  • Update frequent_items to upcoming resp styling

What does this MR do?

The upcoming responsive top nav will need to reuse the frequent_items components in a different context than they originally exist.

This MR adds some flexibility to the classes applied in these components so that:

• They can be reused outside the .dropdown-menu context.
• The search bar can still be shown in a responsive view.

Screenshots (strongly suggested)

Description	Before	After
With Feature.enable(:combined_menu) - items still look right
With Feature.disable(:combined_menu) - items still look right
With Feature.enable(:combined_menu) + small display - search bar is now visible. This responsive view will be enhanced in an upcoming MR
With Feature.disable(:combined_menu) + small display - search bar is still hidden

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.