Add changelog, undo null tolerance.

Null tolerance is a plague. I wish `String` did not admit nulls and we had to
use Optional<String> for potentially null strings, but Java will forever be
cursed with the billion dollar mistake.

CHANGELOG.md

@@ -5,6 +5,8 @@
 * Fixed issues with proxy information not getting properly passed through to app.
   Via [#216](//github.com/threerings/getdown/pull/216).
 
+* `appbase` and `latest` properties in `getdown.txt` now process env var subtitutions.
+
 ## 1.8.4 - May 14, 2019
 
 * Added `verify_timeout` config to allow customization of the default (60 second) timeout during

core/src/main/java/com/threerings/getdown/data/Application.java

@@ -586,7 +586,7 @@ public class Application
             throw new RuntimeException("m.missing_appbase");
         }
 
-        // check if we're overriding the domain in the appbase
+        // check if we're overriding the domain in the appbase, and sub envvars
         _appbase = processArg(SysProps.overrideAppbase(_appbase));
 
         // make sure there's a trailing slash
@@ -606,8 +606,9 @@ public class Application
         }
 
         // check for a latest config URL
-        String latest = processArg(config.getString("latest"));
+        String latest = config.getString("latest");
         if (latest != null) {
+            latest = processArg(latest);
             if (latest.startsWith(_appbase)) {
                 latest = _appbase + latest.substring(_appbase.length());
             } else {
@@ -1121,10 +1122,6 @@ public class Application
     /** Replaces the application directory and version in any argument. */
     protected String processArg (String arg)
     {
-        if (arg == null) {
-            return null;
-        }
-
         arg = arg.replace("%APPDIR%", getAppDir().getAbsolutePath());
         arg = arg.replace("%VERSION%", String.valueOf(_version));