Add proxy support based on environment variables (#1855)

We pull proxy settings from environment variables:

- HTTPS_PROXY for sending, profile pulls, and attachment download/upload
- WSS_PROXY for connecting to the websocket for receiving messages
- ALL_PROXY to provide one server for both

More details on our proxy handling:

- https://github.com/Rob--W/proxy-from-env#environment-variables
- https://github.com/TooTallNate/node-proxy-agent

This is the natural way of things for Linux. My understanding is that
most proxies on MacOS are system-wide and transparent, so it's not so
urgent. But Windows will likely require further UI for configuration.
Will need to do some testing with Windows users.

js/libtextsecure.js

@@ -37461,7 +37461,19 @@ var TextSecureServer = (function() {
     }
 
     function createSocket(url) {
-      var requestOptions = { ca: window.config.certificateAuthorities };
+      var proxyUrl = window.getProxyForUrl(url);
+      var requestOptions;
+      if (proxyUrl) {
+        console.log('createSocket: using proxy url', proxyUrl);
+        requestOptions = {
+            agent: ProxyAgent(proxyUrl)
+        };
+      } else {
+        requestOptions = {
+            ca: window.config.certificateAuthorities,
+        };
+      }
+
       return new nodeWebSocket(url, null, null, null, requestOptions);
     }
 
@@ -37475,12 +37487,23 @@ var TextSecureServer = (function() {
         console.log(options.type, url);
         var timeout = typeof options.timeout !== 'undefined' ? options.timeout : 10000;
 
+        var proxyUrl = window.getProxyForUrl(url);
+        var agent;
+        if (proxyUrl) {
+            console.log('promixe_ajax: using proxy url', proxyUrl);
+            agent = new ProxyAgent(proxyUrl);
+        } else {
+            agent = new httpsAgent({
+                ca: options.certificateAuthorities
+            });
+        }
+
         var fetchOptions = {
           method: options.type,
           body: options.data || null,
           headers: { 'X-Signal-Agent': 'OWD' },
-          agent: new httpsAgent({ca: options.certificateAuthorities}),
-          timeout: timeout
+          agent: agent,
+          timeout: timeout,
         };
 
         if (fetchOptions.body instanceof ArrayBuffer) {