Update list of proxy authentication URLs

[dstillman]

https://forums.zotero.org/discussion/66667/zotero-5-proxy-configuration-via-pac-file-failure-to-authenticate-regression

[dstillman]

Current list:

http://www.acm.org
http://www.ebscohost.com
http://www.elsevier.com
http://www.ieee.org
http://www.jstor.org
http://www.ovid.com
http://www.springer.com
http://www.tandfonline.com

Changes:

• HTTP -> HTTPS where possible
• http://www.elsevier.com -> https://www.sciencedirect.com/

[dstillman]

And to explain the purpose of this, most/all HTTP requests in Zotero are made as background requests so that they don't trigger random popups at unexpected times (e.g., when something is being saved from the connector), but we need to make a foreground request to trigger proxy authentication for proxies that require it. So if we detect that a proxy is configured, we pick three URLs from this list at startup and make foreground requests to them to trigger the authentication prompt.