- Fix: Ensure PHP files are present in dist/extensions alongside JavaScript files using them

- Fix: Bug in obtaining `extPath` in ext-server_opensave.js
- Enhancement: Add config `avoidClientSide` to avoid using client-side support by default (and always require server)
- Build: Require Node 8.5

dist/extensions/savefile.php

@@ -0,0 +1,16 @@
+<?php
+  // You must first create a file "savefile_config.php" in this extensions directory and do whatever
+  //   checking of user credentials, etc. that you wish; otherwise anyone will be able to post SVG
+  //   files to your server which may cause disk space or possibly security problems
+  require('savefile_config.php');
+  if (!isset($_POST['output_svg'])) {
+    print 'You must supply output_svg';
+    exit;
+  }
+  $svg = $_POST['output_svg'];
+  $filename = (isset($_POST['filename']) && !empty($_POST['filename']) ? preg_replace('@[\\\\/:*?"<>|]@u', '_', $_POST['filename']) : 'saved') . '.svg'; // These characters are indicated as prohibited by Windows
+
+  $fh = fopen($filename, 'w') or die("Can't open file");
+  fwrite($fh, $svg);
+  fclose($fh);
+?>