R1:03ebbccbc99f

Restore image proxying to Remarkup

Summary:
Previously, Remarkup allowed you to paste in an image URI and get an inline
image. However, it did this by hotlinking the image which isn't so hot in an
open source product.

Restore this feature, but use image proxying instead. The existing image macro
code does most of the work.

There is a mild security risk depending on the network setup so I've left this
default-disabled and made a note about it. It should be safe to enable for
Facebook.

Test Plan:
Pasted…