Add the invite members to the quick menu

What does this MR do?

• adds Invite members to quick link in the navbar
• adds tracking for the clicking of the link
• uses experiment control group for recording in vs out of the experiment
• adds experiment for icon usage with Invite members

No changelog since the feature is behind the Gitlab::Experimentation.active?(:invite_members_new_dropdown) check, which ensures the experiment/feature is enabled before implementing the link.

Testing locally

• To test showing this at all, set the experiment to something low, but with a percentage greater than 0 in the rails console like so:

Feature.enable_percentage_of_time(:invite_members_new_dropdown_experiment_percentage, 1)

• Visit a project or group.
• Click + quick link in the navbar.
• To test the experimentation variant, set the experiment to 💯 in the rails console like so(may need to clear cache, or reload multiple times if trying locally with a lower percentage first):

Feature.enable_percentage_of_time(:invite_members_new_dropdown_experiment_percentage, 100)

Screenshots (strongly suggested)

While inside a project

without icon	with icon

While inside a group

without icon	with icon

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

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
• 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

Related to #268129