Don't use inline script tag to pass pref_code to JS

This also simplifies the code a bit, as the value of the preference data
was never used so doesn't need to be included now. Primary reason for
this change is to work towards disabling inline JS with a CSP header.
There's still more stuff to fix before the CSP header can be applied,
though.
d6: Don't include non-HTTPS images into the page

So I had to move the images to s.vndb.org, which is, I suppose, where
they belong anyway.
Don't use inline script to pass screenshot sizes in VN edit

Same reasoning as 0d191b5cd8c79eef3cef63c40ded96506a40382a
Replace some 'onclick' attributes with JS-generated events

I think this was the last piece of inline JS.
Handler::VNEdit: Fix 500 when editing VN without screenshots

Broken in 44b11883da3a7e6f1138984ceeba162e0f90dc35
Completely drop l_vnn column from VN info

Used to link to a visual-novels.net review or something. Links have been
hidden and dead since ages. No need to keep the column around.
Minor doc changes