Add support for toast messages on user actions in Requirements

What does this MR do?

This feature is currently behind a feature flag which can be enabled via Rails console for a specific project as;

Feature.enable(:requirements_management, Project.find_by_full_path('gitlab-org/gitlab-shell'))

This MR adds following changes to Requirements Management;

• Show toast messages for confirmation of actions like Create, Edit, Archive and Reopen.
• Adds missing license check in ee/app/views/layouts/nav/_requirements_link.html.haml to ensure that Requirements nav sidebar link is only visible on eligible license.

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

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

#207981 (closed) #207954 (closed) #210306 (closed)