Fix compatibility with authenticated proxies

It's not clear when this became an issue, but our usual background HTTP
requests (set that way to avoid triggering auth prompts when saving from
websites) weren't triggering proxy authentication dialogs, which was breaking
most network activity in Standalone. To fix this, we now make a foreground
request at startup to a file on S3 and resolve the Zotero.proxyAuthComplete
promise when we're done. Any network requests that want to wait for proxy
authentication can wait for that promise.

This behavior can be disabled via the triggerProxyAuthentication hidden pref.