Move attachments to static directory

[thomasrebele]

The attachments should not be a part of the themes directory.

[zhangbutao]

Move attachments to static directory

Why should the static keyword be retained? Isn't it enough to just use attachments?
I'm really bad at naming things, haha. cc @zabetak Any suggesstion about the directory naming?

[zabetak]

@zhangbutao I don't have a strong preference on where to put the attachments. If the new location is better for some reason then I am OK to move them there.

[thomasrebele]

The static is defined by the Hugo directory structure. If we move the attachments to the root, they would not get copied to the public folder, and accessing them in the browser would result in a Page Not Found (I just verified this to be sure. If you want to try, don't forget to delete the public folder to remove the previous build).

I've fixed the issues pointed out in @zhangbutao's comment. I hope the PR will get accepted, as it is a necessary preparation for a link-validation PR that I'm working on (#94).

[zhangbutao]

+1

[zhangbutao]

Merged. Thanks @thomasrebele @zabetak