FEATURE: Implement 2factor login TOTP

implemented review items.

Blocking previous codes - valid 2-factor auth tokens can only be authenticated once/30 seconds.
I played with updating the “last used” any time the token was attempted but that seemed to be overkill, and frustrating as to why a token would fail.
Translatable texts.
Move second factor logic to a helper class.
Move second factor specific controller endpoints to its own controller.
Move serialization logic for 2-factor details in admin user views.
Add a login ember component for de-duplication
Fix up code formatting
Change verbiage of google authenticator

add controller tests:
second factor controller tests
change email tests
change password tests
admin login tests

add qunit tests - password reset, preferences

fix: check for 2factor on change email controller
fix: email controller - only show second factor errors on attempt
fix: check against 'true' to enable second factor.

Add modal for explaining what 2fa with links to Google Authenticator/FreeOTP

add two factor to email signin link

rate limit if second factor token present

add rate limiter test for second factor attempts

test/javascripts/helpers/create-pretender.js.es6

@@ -227,6 +227,16 @@ export default function() {
                           current_email: 'current@example.com' });
       }
 
+      if (data.password === 'need-second-factor') {
+        if (data.second_factor_token) {
+          return response({username: 'eviltrout'});
+        }
+        return response({ error: "Invalid Second Factor",
+                          reason: "invalid_second_factor",
+                          sent_to_email: 'eviltrout@example.com',
+                          current_email: 'current@example.com' });
+      }
+
       return response(400, {error: 'invalid login'});
     });