FEATURE: per client user tokens

Revamped system for managing authentication tokens.

- Every user has 1 token per client (web browser)
- Tokens are rotated every 10 minutes

New system migrates the old tokens to "legacy" tokens,
so users still remain logged on.

Also introduces weekly job to expire old auth tokens.

db/migrate/20170124181409_add_user_auth_tokens.rb

@@ -0,0 +1,43 @@
+class AddUserAuthTokens < ActiveRecord::Migration
+  def down
+    add_column :users, :auth_token, :string
+    add_column :users, :auth_token_updated_at, :datetime
+    execute <<SQL
+      UPDATE users
+        SET auth_token = user_auth_tokens.auth_token,
+            auth_token_updated_at = user_auth_tokens.created_at
+      FROM user_auth_tokens
+      WHERE legacy AND user_auth_tokens.user_id = users.id
+SQL
+
+    drop_table :user_auth_tokens
+
+  end
+
+  def up
+    create_table :user_auth_tokens do |t|
+      t.integer :user_id, null: false
+      t.string  :auth_token, null: false
+      t.string  :prev_auth_token, null: false
+      t.string  :user_agent
+      t.boolean :auth_token_seen, default: false, null: false
+      t.boolean :legacy, default: false, null: false
+      t.inet    :client_ip
+      t.datetime :rotated_at, null: false
+      t.timestamps
+    end
+
+    add_index :user_auth_tokens, [:auth_token]
+    add_index :user_auth_tokens, [:prev_auth_token]
+
+    execute <<SQL
+    INSERT INTO user_auth_tokens(user_id, auth_token, prev_auth_token, legacy, created_at, rotated_at)
+    SELECT id, auth_token, auth_token, true, auth_token_updated_at, auth_token_updated_at
+    FROM users
+    WHERE auth_token_updated_at IS NOT NULL AND auth_token IS NOT NULL
+SQL
+
+    remove_column :users, :auth_token
+    remove_column :users, :auth_token_updated_at
+  end
+end