Added length rule for index files [warn]

- index.js files are meant to be an index, not contain behaviour or logic
- files longer than 50 lines are indicative of code in the wrong place, all though not definitive
- enabling this as a hint to get us to move code to better locations

.eslintrc.js

@@ -16,6 +16,12 @@ module.exports = {
         'one-var': [2, 'never']
     },
     overrides: [
+        {
+            files: '**/index.js',
+            rules: {
+                'max-lines': ['warn', {skipBlankLines: true, skipComments: true, max: 50}]
+            }
+        },
         {
             files: 'core/server/api/canary/*',
             rules: {