Proxy support

[Mikaela]

I am interested in seeing support for especially SOCKS proxies for Tor.

• Desktop: https://github.com/vector-im/riot-web/issues/3320
• Android: Support connecting via SOCKS proxy (Tor) riot-android#1234
• iOS: TOR needs to be integrated in IOS version because TOR is BROKEN with IOS element-ios#1085

I am opening this issue, because I think this would be the most logical issue tracker and I didn't find any issue here already while a few searches in the other repositories brought the issues above.