Revocable sessions (#3616)

* feat: Revocable sessions

* fix: Tests using sign_in

* feat: Configuration entry for the maximum number of session activations

db/migrate/20170623152212_create_session_activations.rb

@@ -0,0 +1,13 @@
+class CreateSessionActivations < ActiveRecord::Migration[5.1]
+  def change
+    create_table :session_activations do |t|
+      t.integer :user_id,   null: false
+      t.string :session_id, null: false
+
+      t.timestamps
+    end
+
+    add_index :session_activations, :user_id
+    add_index :session_activations, :session_id, unique: true
+  end
+end