add ver get parameter for js/css files to improve cache behaviour

add ver get parameter for js/css files to improve cache behaviour

we do not set an Expire header for our js files, and certain browsers
do not even make the request then (on which we would answer
with a correct 304 not modified)

so to force the browser to load a new version of the gui when we change
something, we add the package version as a get parameter

when doing this, the browsers still cache the file, until the get
parameter changes, which is exactly what we want

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Reviewed-by: Thomas Lamprecht <t.lamprecht@proxmox.com>