publish sharing links

[bones7242]

User story: As a user, after I publish an image, I want to be shown the direct link to that image and be able to copy it easily, so that I can share or embed it elsewhere on the internet.

After a publish is complete, it should display links/buttons for sharing the published asset.

Currently the copy-to-clipboard button is broken. That needs to be fixed.

It should also offer 'tweet', 'post to reddit', embedding options (like an image tag using your spee.ch url), and other helpful sharing buttons.

[bones7242]

note: publish finish could/should redirect to 'show' route.

[bones7242]

This one will be fast after #66 is closed