Add the file operations urls to admin site.
Improve template file _browser.html

1. Add quotes so that the httpreqeust post method can send and correct value.
   Otherwise, content after space will be omitted.

2. Show upload and mkdir form if there is no objects.
Added class member "modified_by" to CloudObject class.
Add error handling, message info and logging to upload, mkdir, delete, move and rename.
Add rename and move admin template files.
Wrap rename and move templates with admin.
Add AWS_S3_CACHE_SECONDS