Add 'sourceid' settings var specifically for skin-files insertion.

Add logic after (both!) LLTransUtil::parseStrings() calls to ensure that
"[sourceid]" embedded in (e.g.) strings.xml content will be replaced.

indra/newview/app_settings/settings.xml

@@ -12285,6 +12285,17 @@
       <key>Value</key>
       <integer>0</integer>
     </map>
+    <key>sourceid</key>
+    <map>
+      <key>Comment</key>
+      <string>Identify referring agency to Linden web servers</string>
+      <key>Persist</key>
+      <integer>1</integer>
+      <key>Type</key>
+      <string>String</string>
+      <key>Value</key>
+      <string />
+    </map>
     <key>SpeakerParticipantDefaultOrder</key>
     <map>
       <key>Comment</key>

indra/newview/llappviewer.cpp

@@ -771,6 +771,8 @@ bool LLAppViewer::init()
 	LLUI::setupPaths();
 	LLTransUtil::parseStrings("strings.xml", default_trans_args);
 	LLTransUtil::parseLanguageStrings("language_settings.xml");
+	// parseStrings() sets up the LLTrans substitution table. Add this one item.
+	LLTrans::setDefaultArg("[sourceid]", gSavedSettings.getString("sourceid"));
 
 	// Setup notifications after LLUI::setupPaths() has been called.
 	LLNotifications::instance();
@@ -2242,6 +2244,8 @@ bool LLAppViewer::initConfiguration()
 	LLUI::setupPaths(); // setup paths for LLTrans based on settings files only
 	LLTransUtil::parseStrings("strings.xml", default_trans_args);
 	LLTransUtil::parseLanguageStrings("language_settings.xml");
+	// parseStrings() sets up the LLTrans substitution table. Add this one item.
+	LLTrans::setDefaultArg("[sourceid]", gSavedSettings.getString("sourceid"));
 	// - set procedural settings
 	// Note: can't use LL_PATH_PER_SL_ACCOUNT for any of these since we haven't logged in yet
 	gSavedSettings.setString("ClientSettingsFile",