Implement auto-complete for /admin/ forms that reference User

We have so many users now that loading these forms take forever.
Instead, implement a textbox with autocomplete using django-selectable,
which will not load the whole list of users at once.

pgweb/core/admin.py

@@ -1,14 +1,33 @@
+from django import forms
 from django.contrib import admin
 
+from selectable.forms.widgets import AutoCompleteSelectMultipleWidget
+
 from pgweb.core.models import Version, OrganisationType, Organisation
 from pgweb.core.models import ImportedRSSFeed, ImportedRSSItem
 from pgweb.core.models import ModerationNotification
 
+from pgweb.core.lookups import UserLookup
+
+class OrganisationAdminForm(forms.ModelForm):
+	class Meta:
+		model = Organisation
+		exclude = ()
+		widgets = {
+			'managers': AutoCompleteSelectMultipleWidget(lookup_class=UserLookup),
+		}
+
+	def __init__(self, *args, **kwargs):
+		super(OrganisationAdminForm, self).__init__(*args, **kwargs)
+		self.fields['managers'].widget.can_add_related = False
+		self.fields['managers'].widget.can_change_related = False
+		self.fields['managers'].widget.can_delete_related = False
+
 class OrganisationAdmin(admin.ModelAdmin):
+	form = OrganisationAdminForm
 	list_display = ('name', 'approved', 'lastconfirmed',)
 	list_filter = ('approved',)
 	ordering = ('name', )
-	filter_horizontal = ('managers', )
 	search_fields = ('name', )
 
 class VersionAdmin(admin.ModelAdmin):