Add button to synchronize all execution environments

This adds a button to the execution environment index page that, when
clicked, causes all execution environments to be synchronized to the
runner management (Poseidon) by creating or replacing them.

CodeOcean does not synchronize it's execution environments on startup or
when a new runner management configuration is used for the first time.
The administrator has to manually start this process by pressing this
button.

The equivalent for syncing just one execution environment is updating
it.

app/policies/execution_environment_policy.rb

@@ -8,4 +8,8 @@ class ExecutionEnvironmentPolicy < AdminOnlyPolicy
   [:index?].each do |action|
     define_method(action) { admin? || teacher? }
   end
+
+  def synchronize_all_to_poseidon?
+    admin?
+  end
 end