Using bower to manage admin-lte resource and remove AdminLTE directory

Steps:
1. Using bower to manage AdminLTE resource: bower install admin-lte --save
2. Modify admin_lte.libraries.yml to using the new resource
3. Remove the old AdminLTE directory
4. Write assets/js/admin_lte.script.js according to bower_components/AdminLTE/dist/js/app.js