Move contribution url into centralised place

Using the work by @mingness to simplify the contributions workflow.
Disabled the post request because that data is not saved anywhere and most hosts do not support it

app/build.gradle.kts

@@ -17,6 +17,7 @@ group = rootProject.group
 tasks.withType<JavaExec> {
     systemProperty("processing.version", version)
     systemProperty("processing.revision", "1296")
+    systemProperty("processing.contributions.source", "https://contributions-preview.processing.org/contribs.txt")
 }
 
 

app/src/processing/app/contrib/ContributionListing.java

@@ -44,7 +44,7 @@ public class ContributionListing {
    * Stable URL that will redirect to wherever the file is hosted.
    * Changed to use https in 4.0 beta 8 (returns same data).
    */
-  static final String LISTING_URL = "https://download.processing.org/contribs";
+  static final String LISTING_URL = System.getProperty("processing.contributions.source","https://download.processing.org/contribs");
   static final String LOCAL_FILENAME = "contribs.txt";
 
   /** Location of the listing file on disk, will be read and written. */

app/src/processing/app/contrib/ContributionManager.java

@@ -67,10 +67,10 @@ public class ContributionManager {
       conn.setConnectTimeout(15 * 1000);
       conn.setReadTimeout(60 * 1000);
 
-      if (post == null) {
+      // Disable the post for now, since it's not used
+      if (post == null || System.getProperty("processing.contributions.source") != null) {
         conn.setRequestMethod("GET");
         conn.connect();
-
       } else {
         post = Util.gzipEncode(post);
         conn.setRequestMethod("POST");