Copy execution environment to Poseidon on create and update

When creating or updating an execution environment, an API call to
Poseidon is made with the necessary information to create the
corresponding Nomad job.

If runner management is configured, his will display a warning
(currently in the same color as if it were a success) in the UI, if the
API call fails. The environment is saved even if it fails.
If runner management is not configured, this warning will not be created.

db/migrate/20210601095654_add_cpu_limit_to_execution_environment.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddCpuLimitToExecutionEnvironment < ActiveRecord::Migration[6.1]
+  def change
+    add_column :execution_environments, :cpu_limit, :integer, default: 20
+  end
+end