Fix to mark Gfycat'ed links as visited

This seems to fix gif links that are turned to gfycat videos not turning purple.

Notes:
- I only tested this on Chrome, sorry!
- Only researched the showImage.js file for a short time, so I don't know if this is the right way to do this.
  Should I use BrowserStrategy.addURLToHistory instead?
- I know technically we haven't visited the original gif, but I think it's expected to turn visited if we've enabled the option to use gfycat instead.