Remove support for deprecated password-hashing algorithm

* Remove all methods implementing and checking for the old algorithm
* Document a pre-upgrade step to check for remaining obsolete passwords
* Remove config.salt

test/controllers/preferences_controller_test.rb

@@ -2,11 +2,6 @@ require 'test_helper'
 
 class PreferencesControllerTest < ActionController::TestCase
 
-  def setup
-    super
-    assert_equal "change-me", Tracks::Config.salt
-  end
-
   test "render_date_format requires login" do
     get :render_date_format
     assert_redirected_to login_path

test/controllers/users_controller_test.rb

@@ -18,7 +18,7 @@ class UsersControllerTest < ActionController::TestCase
     get :index
     assert_response :success
     assert_equal "TRACKS::Manage Users", assigns['page_title']
-    assert_equal 5, assigns['total_users']
+    assert_equal 4, assigns['total_users']
     assert_equal users_url, session['return-to']
   end