users: track logins, signups, and other user events.

Add tracking of certain important user actions. These events include:

* Logins
* Logouts
* Failed login attempts
* Account creations
* Account deletions
* Password reset requests
* Password changes
* Email address changes

This is similar to the mod actions log, except for account activity
related to a single user.

The information tracked includes the user, the event type (login,
logout, etc), the timestamp, the user's IP address, IP geolocation
information, the user's browser user agent, and the user's session ID
from their session cookie. This information is visible to mods only.

This is done with three models. The UserEvent model tracks the event
type (login, logout, password change, etc) and the user. The UserEvent
is tied to a UserSession, which contains the user's IP address and
browser metadata. Finally, the IpGeolocation model contains the
geolocation information for IPs, including the city, country, ISP, and
whether the IP is a proxy.

This tracking will be used for a few purposes:

* Letting users view their account history, to detect things like logins
  from unrecognized IPs, failed logins attempts, password changes, etc.
* Rate limiting failed login attempts.
* Detecting sockpuppet accounts using their login history.
* Detecting unauthorized account sharing.

test/functional/passwords_controller_test.rb

@@ -20,6 +20,7 @@ class PasswordsControllerTest < ActionDispatch::IntegrationTest
         assert_redirected_to @user
         assert_equal(false, @user.reload.authenticate_password("12345"))
         assert_equal(@user, @user.authenticate_password("abcde"))
+        assert_equal(true, @user.user_events.password_change.exists?)
       end
 
       should "update the password when given a valid login key" do
@@ -29,6 +30,7 @@ class PasswordsControllerTest < ActionDispatch::IntegrationTest
         assert_redirected_to @user
         assert_equal(false, @user.reload.authenticate_password("12345"))
         assert_equal(@user, @user.authenticate_password("abcde"))
+        assert_equal(true, @user.user_events.password_change.exists?)
       end
 
       should "allow the site owner to change the password of other users" do
@@ -55,6 +57,7 @@ class PasswordsControllerTest < ActionDispatch::IntegrationTest
         assert_response :success
         assert_equal(@user, @user.reload.authenticate_password("12345"))
         assert_equal(false, @user.authenticate_password("abcde"))
+        assert_equal(false, @user.user_events.password_change.exists?)
       end
 
       should "not update the password when password confirmation fails for the new password" do