AbsolutePaths: initial work to allow Etherpad to be run without changing CWD

With this change, it is no longer necessary to "cd" to the Etherpad base
directory to start it: Etherpad runs from everywhere.

Known issues:
- unless the program is started as before (CWD == base directory) it is still
  not possible to install & uninstall plugins via the web interface

--HG--
branch : absolute-paths

src/static/js/pluginfw/plugins.js

@@ -5,6 +5,7 @@ var fs = require("fs");
 var tsort = require("./tsort");
 var util = require("util");
 var _ = require("underscore");
+var settings = require('../../../node/utils/Settings');
 
 var pluginUtils = require('./shared');
 
@@ -95,7 +96,7 @@ exports.update = async function () {
 
 exports.getPackages = async function () {
   // Load list of installed NPM packages, flatten it to a list, and filter out only packages with names that
-  var dir = path.resolve(npm.dir, '..');
+  var dir = settings.root;
   let data = await util.promisify(readInstalled)(dir);
 
   var packages = {};