Name conflicts under static folder when doing veripress generate

[momocow]

When doing veripress generate, sub-folders of themes' static folder cannot have the same name as sub-folders of user's static folder.

For example, given a theme has its images under themes/<theme_name>/static/img. Meanwhile, the user already has his images under static/img.

If running veripress generate, FileExistsError will be thrown. Rename one of img folders to images fix the error.