Support EXPLAIN on Greenplum. Fixes #3097

- Extract SQLEditor.execute and SQLEditor._poll into their own files and add test around them
 - Extract SQLEditor backend functions that start executing query to their own files and add tests around it
 - Move the Explain SQL from the front-end and now pass the Explain plan parameters as a JSON object in the start query call.
 - Extract the compile_template_name into a function that can be used by the different places that try to select the version of the template and the server type

web/regression/javascript/fake_endpoints.js

@@ -8,5 +8,9 @@
 //////////////////////////////////////////////////////////////////////////
 
 define(function () {
-  return {'static': '/base/pgadmin/static/<path:filename>'};
+  return {
+    'static': '/base/pgadmin/static/<path:filename>',
+    'sqleditor.poll': '/sqleditor/query_tool/poll/<path:trans_id>',
+    'sqleditor.query_tool_start': '/sqleditor/query_tool/start/<path:trans_id>'
+  };
 });