Bug 1222542 - Run eslint against all of the ui/ directory

Since previously the JS files under ui/partials/ and ui/plugins/ were
omitted. We now need to also explicitly exclude the vendor directory,
which we can do using `.eslintignore`.

This also means that if people prefer to run eslint directly, they
can use `eslint ui/` rather than having to type out the multiple
directories used previously.

.eslintignore

@@ -0,0 +1 @@
+ui/vendor/

Gruntfile.js

@@ -259,9 +259,7 @@ module.exports = function(grunt) {
             options: {
                 config: '.eslintrc'
             },
-            target: ['ui/js/*.js',
-                     'ui/js/**/*.js',
-                     'ui/js/**/**/*.js']
+            target: ['ui/']
         }
     });