add browser cache workaround

add browser cache workaround

to prevent indefinite caching of the js files in the browser,
we add the package version to the js files, this will be read
by the pmgproxy and added as template variables

we use those to add the version to the get parameter and
thus tell the browser the file has changed

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>