Modify InvitationAdmin to use autocomplete widget for inviter field

Replace raw_id_fields with autocomplete_fields for the inviter field, as
that's more practical when searching for a user, although it may affect
performance of the form if there is a large number of users.

Fixes #259.

invitations/admin.py

@@ -14,7 +14,7 @@
 @admin.register(Invitation)
 class InvitationAdmin(admin.ModelAdmin):
     list_display = ("email", "sent", "accepted")
-    raw_id_fields = ("inviter",)
+    autocomplete_fields = ["inviter"]
 
     def get_form(self, request, obj=None, **kwargs):
         if obj: