Resolves #14370 -- Adds select2 widget for foreign keys in admin

Adds jQuery Select2 version 4 to support async select inputs
including a search feature.

Changes:
- only str representation is supported at this point
- the search feature uses the same field as the model admin

.eslintignore

@@ -1,3 +1,4 @@
-**/{*.min,jquery}.js
+**/*.min.js
+**/vendor/**/*.js
 django/contrib/gis/templates/**/*.js
 node_modules/**.js

django/contrib/admin/options.py

@@ -19,6 +19,7 @@
     NestedObjects, flatten_fieldsets, get_deleted_objects,
     lookup_needs_distinct, model_format_dict, quote, unquote,
 )
+from django.contrib.admin.widgets import AdminForeignKeyWidget
 from django.contrib.auth import get_permission_codename
 from django.core.exceptions import (
     FieldDoesNotExist, FieldError, PermissionDenied, ValidationError,
@@ -97,6 +98,7 @@ class IncorrectLookupParameters(Exception):
 class BaseModelAdmin(six.with_metaclass(forms.MediaDefiningClass)):
     """Functionality common to both ModelAdmin and InlineAdmin."""
 
+    autocomplete_fields = ()
     raw_id_fields = ()
     fields = None
     exclude = None
@@ -213,6 +215,14 @@ def formfield_for_foreignkey(self, db_field, request, **kwargs):
         Get a form Field for a ForeignKey.
         """
         db = kwargs.get('using')
+
+        if db_field.name in self.autocomplete_fields:
+            related_admin = self.admin_site._registry.get(db_field.remote_field.model)
+            if not related_admin.search_fields:
+                msg = 'The admin for %s needs to implement "search_fields".'
+                raise NotImplementedError(msg % db_field.remote_field.model)
+            kwargs['widget'] = AdminForeignKeyWidget(model_admin=self, using=db)
+
         if db_field.name in self.raw_id_fields:
             kwargs['widget'] = widgets.ForeignKeyRawIdWidget(db_field.remote_field,
                                     self.admin_site, using=db)

django/contrib/admin/sites.py

@@ -3,6 +3,7 @@
 from django.apps import apps
 from django.conf import settings
 from django.contrib.admin import ModelAdmin, actions
+from django.contrib.admin.views.main import ForeignKeyJsonView
 from django.contrib.auth import REDIRECT_FIELD_NAME
 from django.core.exceptions import ImproperlyConfigured, PermissionDenied
 from django.db.models.base import ModelBase
@@ -239,6 +240,7 @@ def wrapper(*args, **kwargs):
             url(r'^jsi18n/$', wrap(self.i18n_javascript, cacheable=True), name='jsi18n'),
             url(r'^r/(?P<content_type_id>\d+)/(?P<object_id>.+)/$', wrap(contenttype_views.shortcut),
                 name='view_on_site'),
+            url(r'^foreignkey_json/$', wrap(ForeignKeyJsonView.as_view(admin_site=self)), name='fk_json')
         ]
 
         # Add in each model's views, and create a list of valid URLS for the

django/contrib/admin/static/admin/css/vendor/select2/LICENSE.md

@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2012-2015 Kevin Brown, Igor Vaynberg, and Select2 contributors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.