Feature: opt-in survey when exiting a game

On first start-up, the game will ask if you want to participate
in our automated survey. You have to opt-in, and can easily opt-out
(via the Options) at any time.

When opt-in, whenever you exit a game, a JSON blob will be send
to the survey server hosted by OpenTTD. This JSON blob contains
information that gives a global picture of the game just played:
- What settings were used
- How many humans vs AIs
- How long the game has been played
- Basic information about the OS / CPU

All this information is kept very generic, so there is no
chance we send private information to our survey server.
Nothing in the JSON blob could identify you as a person; it
mostly tells about the game played. At any time you can see
what the JSON blob includes, by pressing the "Preview Survey
Results" button in-game.

src/network/core/config.cpp

@@ -66,3 +66,13 @@ const char *NetworkContentMirrorUriString()
 {
 	return GetEnv("OTTD_CONTENT_MIRROR_URI", "https://binaries.openttd.org/bananas");
 }
+
+/**
+ * Get the URI string for the survey from the environment variable OTTD_SURVEY_URI,
+ * or when it has not been set a hard coded URI of the production server.
+ * @return The survey's URI string.
+ */
+const char *NetworkSurveyUriString()
+{
+	return GetEnv("OTTD_SURVEY_URI", "https://survey-participate.openttd.org/");
+}