Add new private proc 'attachments::get_attachments_url', and use it to complement the logic of 'attachments::get_url'
As 'attachments::get_url' returns the value of the attachments package 'RelativeUrl' parameter, which can change at any time, it could happen that previously mounted attachments have a different url and are not found anymore.
We try our best here to find a mounted attachments package under 'base_url' to mitigate this, probably flawed, package logic.
In the future, probably a better method should be used for URL resolving that is not so broken.
The whole thing is even more weird, as the attachments package is currently a singleton that auto-mounts on /attachments, so i am tempted to replace this whole thing with just that, but anyway...