Django-adminlte-ui

- fix 2 get 500 when exchange menu;
- add .gitattributes.

- add exchange_menu logic with permission limit;
- add message when call exchange menu;
- update docs;
- update models, content_type option add null=True;
- add js logic for menu;
- add treebeard as menu depends;
- add select2 into widgets.py;
- update site_logo save path, base on settings.MEDIA_ROOT & settings.MEDIA_URL;
- fix base.html & login.html b tag issue;
- change base.html extra_style and extra_head position.

1. add models with options;
2. add general_option.html to manager general option;
3. setup app label to `django_admin_settings`;
4. register and add general_option into menu;
5. add docs.

fix filter and search issue at change_list.html, consult django-suit.

1. add pages [ delete_selected_confirmation.html，add object_history.html，404.html，500.html]；
2. update layout when fieldset.name is not none；
3. update support default block extrahead extrastyle。