When this checkbox is checked, while uploading docs using fileuploader
the default visibility will be set to "public".

At present all uploaded attachments are private, however in following
cases it makes sense to set default as public:

1. Blog post
2. Item catalog
3. Website item

Considering this we decided to add this configurtion so developers can
specify sane default while user still has control over it using
"Customize form"

Enabled this for "blog post" doctype

Unrelatd change: consistent "form settings" section on DocType and customize
form

(cherry picked from commit e8af6b2c16)