Add option to let the update center connect directly to the update sites

Currently the update center needs a client that uses a browser with activated JavaScript (and no blocked sites) to fetch new updates. According to the discussion in the mailing list this seems to be intentional since Jenkins server instances may have no connection to the internet.

However, it is very confusing for a Jenkins administrator, that the updates depend on the clients JavaScript settings and not on the server settings. There should be at least a note or help text on the corresponding update center page.

And since there are a lot of Jenkins instances that can connect to the internet, there should be a fallback that tries to use a direct connection from the server to the update sites.

Isn't a direct connection also used to fetch the tools like maven, jdk, etc.?