Add a top level notification dot for what's new

What does this MR do?

Closes: gitlab-org/growth/engineering#5386 (closed)

It adds a new notification dot for "what's new", that surfaces on the header help navigation item.

Notes:

1. When the header help dropdown is moved within the user profile dropdown for the mobile layout, the "what's new" item is no longer visible.
2. The notification dot doesn't shift from the header help item to the user profile item. Given that "what's new" doesn't transfer within this nested dropdown this makes sense, and also minimizes that there's a different style of notification icon that means something else on the user profile navigation item (captured in screenshots).
3. To view the feature locally you must enable the following feature flags:

Feature.enable(:whats_new_dropdown)
Feature.enable(:whats_new_drawer)

Screenshots

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