Empty state frontend

In the event there is no Agent available the user should get a notice on the Agent managed clusters landing page

Design: https://gitlab.com/gitlab-org/gitlab/uploads/c7953ecc6282261c80e22f67adb9ab63/000_Kubernetes_Agent_-Empty_state-_No_agent_directory.png

Definition of Done

• Add notification message box to agent landing page
• add "Go to Repository" link

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.