Move cluster agent frontend to Core

What does this MR do and why?

Part of making Cluster Agents available in Core (&6290 (closed)).

These changes will not yet be visible in Core, as there is still a helper method preventing the link to it being rendered:

  # app/helpers/clusters_helper.rb
  def display_cluster_agents?(_clusterable)
    false
  end

  # ee/app/helpers/ee/clusters_helper.rb
  override :display_cluster_agents?
  def display_cluster_agents?(clusterable)
    clusterable.is_a?(Project) && clusterable.feature_available?(:cluster_agents)
  end

We leave this unchanged until everything has been moved, at which point it will serve as a simple way to enable the feature.

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#340355 (closed)