Reuse runners per user and execution environment

Co-authored-by: Jan-Eric Hellenberg <jan-eric.hellenberg@student.hpi.uni-potsdam.de>
Co-authored-by: Maximilian Pass <maximilian.pass@student.hpi.uni-potsdam.de>

db/migrate/20210415064948_create_runners.rb

@@ -0,0 +1,14 @@
+class CreateRunners < ActiveRecord::Migration[5.2]
+  def change
+    create_table :runners do |t|
+      t.string :runner_id
+      t.references :execution_environment
+      t.references :user, polymorphic: true
+      t.integer :time_limit
+      t.float :waiting_time
+      t.datetime :last_used
+
+      t.timestamps
+    end
+  end
+end