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.

spec/controllers/session_controller_spec.rb

@@ -505,8 +505,8 @@ describe SessionController do
           user.reload
 
           expect(session[:current_user_id]).to eq(user.id)
-          expect(user.auth_token).to be_present
-          expect(cookies[:_t]).to eq(user.auth_token)
+          expect(user.user_auth_tokens.count).to eq(1)
+          expect(UserAuthToken.hash_token(cookies[:_t])).to eq(user.user_auth_tokens.first.auth_token)
         end
       end