Implement client-side registration form validation.

Input forms that need validation will have AJAX requests
performed to get validation decisions live.

All but a few important and common form fields perform
generic validation; these will need a back-end handler
in the future in order to have them validated through AJAX requests.

Information is conveyed on focus and blur for both
errors and successes.

common/djangoapps/student/tests/test_create_account.py

@@ -22,7 +22,9 @@ from notification_prefs import NOTIFICATION_PREF_KEY
 from openedx.core.djangoapps.external_auth.models import ExternalAuthMap
 from openedx.core.djangoapps.lang_pref import LANGUAGE_KEY
 from openedx.core.djangoapps.site_configuration.tests.mixins import SiteMixin
-from openedx.core.djangoapps.user_api.accounts import USERNAME_INVALID_CHARS_ASCII, USERNAME_INVALID_CHARS_UNICODE
+from openedx.core.djangoapps.user_api.accounts import (
+    USERNAME_BAD_LENGTH_MSG, USERNAME_INVALID_CHARS_ASCII, USERNAME_INVALID_CHARS_UNICODE
+)
 from openedx.core.djangoapps.user_api.preferences.api import get_user_preference
 from student.models import UserAttribute
 from student.views import REGISTRATION_AFFILIATE_ID, REGISTRATION_UTM_CREATED_AT, REGISTRATION_UTM_PARAMETERS
@@ -476,16 +478,16 @@ class TestCreateAccountValidation(TestCase):
 
         # Missing
         del params["username"]
-        assert_username_error("Username must be minimum of two characters long")
+        assert_username_error(USERNAME_BAD_LENGTH_MSG)
 
         # Empty, too short
         for username in ["", "a"]:
             params["username"] = username
-            assert_username_error("Username must be minimum of two characters long")
+            assert_username_error(USERNAME_BAD_LENGTH_MSG)
 
         # Too long
         params["username"] = "this_username_has_31_characters"
-        assert_username_error("Username cannot be more than 30 characters long")
+        assert_username_error(USERNAME_BAD_LENGTH_MSG)
 
         # Invalid
         params["username"] = "invalid username"

common/djangoapps/student/tests/test_long_username_email.py

@@ -5,6 +5,8 @@ import json
 from django.core.urlresolvers import reverse
 from django.test import TestCase
 
+from openedx.core.djangoapps.user_api.accounts import USERNAME_BAD_LENGTH_MSG
+
 
 class TestLongUsernameEmail(TestCase):
 
@@ -34,7 +36,7 @@ class TestLongUsernameEmail(TestCase):
         obj = json.loads(response.content)
         self.assertEqual(
             obj['value'],
-            "Username cannot be more than 30 characters long",
+            USERNAME_BAD_LENGTH_MSG,
         )
 
     def test_long_email(self):