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

app/assets/javascripts/admin/models/admin-user.js.es6

@@ -168,6 +168,14 @@ const AdminUser = Discourse.User.extend({
     }).catch(popupAjaxError);
   },
 
+  disableSecondFactor() {
+    return ajax("/admin/users/" + this.get('id') + "/disable_second_factor", {
+      type: 'PUT'
+    }).then(() => {
+      this.set('second_factor_enabled', false);
+    }).catch(popupAjaxError);
+  },
+
   refreshBrowsers() {
     return ajax("/admin/users/" + this.get('id') + "/refresh_browsers", {
       type: 'POST'