add image and status to statistics page

[JonPurvis]

This takes the statistics page changes from https://github.com/jgrossi/elephpant.me/pull/107 and brings them more up to date as that PR is over 2 years old now.

This PR:

• Adds the image for each Elephpant
• Includes an indicator as to whether it's in your collection or not:

I've also added a little key at the bottom of the page so you know what the icons mean:

This functionality is for logged in users only. If you're not logged in, you will see no difference.

[JonPurvis]

@teiling88 Back over to you to test 😄

[teiling88]

Thank you for your contribution, the change is already live :-)