Fixup model loading

  - Using get_field_by_name loads the appcache, which is not allowed in
    a models module. Use get_field(False), which only looks at local
    fields and does not require populating the cache. This helps with #177.

  - Do not call get_layout_contenttypes during model loading by making
    it lazy. get_layout_contenttypes does a ContentQuery, which requires
    that the django_content_type exists, which is not true during
    syncdb.  This makes the DEBUG_TOOLBAR_PATCH_SETTINGS workaround in #244,
    #214, #235 unnecessary.