[Xon] Attachment Improvements 2.6.7

A collection of improvements to XF's attachment system.

• Byte-range support for resumable downloads
  • Primarily designed for partial content requests for streaming videos, but will work with any file
  • Dependant on the filesystem /internal_data being local, as the remote file system APIs do not easily support this feature
• Improve drag&drop editor support.
  • Stock XenForo/Froala uses "move" which can result in unexpected content deletion.
  • Tint the active editor's boarder when dragging items over it, and show a file upload symbol over it.
• Option to strip jpeg EXIF data
• SVG support
• Nginx's X-Accel-Redirect
• New Permissions for forum/conversations (Respects global attachment size & count limits):
  • Attachment Size (kb).
  • Maximum Attachment Count.
• Adds support for video stream via partial content requests

SVG Support
Support for SVG attachments to be displayable as normal images.