Add support for partial attachment downloads

Confluence does not currently support partial downloads of attachment content using the standard "Range" header. This means that the download of attachments cannot be resumed if interrupted.

Steps to confirm the current behavior:

1. Locate the full URL of an attachment stored in Confluence, such as:
   http://localhost:8080/confluence/download/attachments/32792/orly.jpg
2. Telnet to the appropriate port on a machine running Confluence 2.10.3
3. Make a partial request by entering the following text:

   GET /confluence/download/attachments/32792/orly.jpg HTTP/1.1
   Range: bytes=0-16
   Host: localhost
   
   
   

4. Only 16 bytes should be returned, instead the whole file is returned.