Fix issues with Underscore in the asset pipeline

FEDX-121

The previous approach for handling NPM assets was
to symlink them into the static directory. This appeared
to cause trouble with the asset pipeline where the files
in question were not installed and then old versions were
picked up instead.

This change instead copies NPM libraries to a new
static directory so that the pipeline can consume them
as with any other file. This new directory is added to
.gitignore so that the files don't get accidentally
checked in.

common/lib/xmodule/xmodule/js/js_test.yml

@@ -45,7 +45,7 @@ lib_paths:
     - common_static/js/vendor/jquery.ui.draggable.js
     - common_static/js/vendor/jquery.cookie.js
     - common_static/js/vendor/json2.js
-    - common_static/js/vendor/underscore-min.js
+    - common_static/common/js/vendor/underscore.js
     - common_static/js/vendor/backbone-min.js
     - common_static/js/vendor/jquery.leanModal.js
     - common_static/js/vendor/CodeMirror/codemirror.js

common/static/common/js/spec/main_requirejs.js

@@ -22,7 +22,7 @@
             'jquery.url': 'js/vendor/url.min',
             'sinon': 'js/vendor/sinon-1.17.0',
             'text': 'js/vendor/requirejs/text',
-            'underscore': 'js/vendor/underscore-min',
+            'underscore': 'common/js/vendor/underscore',
             'underscore.string': 'js/vendor/underscore.string.min',
             'backbone': 'js/vendor/backbone-min',
             'backbone.associations': 'js/vendor/backbone-associations-min',

common/static/js/vendor/underscore-min.js

@@ -1 +0,0 @@
-../../../../node_modules/underscore/underscore-min.js

common/static/js_test.yml

@@ -33,7 +33,7 @@ lib_paths:
     - js/vendor/jasmine-imagediff.js
     - js/vendor/jquery.truncate.js
     - js/vendor/mustache.js
-    - js/vendor/underscore-min.js
+    - common/js/vendor/underscore.js
     - js/vendor/underscore.string.min.js
     - js/vendor/backbone-min.js
     - js/vendor/jquery.timeago.js