Merge branch 'fix-invalid-filename-eslint' into 'master'

eslint: Fix invalid filename validation

Attempt to fix the following error which is appearing in every new MR since `eslint-plugin-filenames` was introduced.

```
/builds/gitlab-org/gitlab-ce/app/assets/javascripts/gl_field_error.js.es6
  2:1  error  Filename 'gl_field_error.js.es6' does not match the naming convention  filenames/match-regex
```

This was happening because our ES6 files have the following structure `gl_hello_world.js.es6`. So `eslint-plugin-filenames` was considering as base name `gl_hello_world.js` which didn't pass the previous regex `^[a-z_]+$`

This new regex allows the following filenames to pass as valid:

- `gl_hello_world.js.es6` which base name translates to `gl_hello_world.js` 
-  `gl_foo_bar.js` which base name translates to `gl_foo_bar`
-  `d3.js` which base name translates to `d3` (especial case, maybe we should move this to the vendor dir and make the regex more specific)

See merge request !7281

.eslintrc

@@ -4,7 +4,7 @@
     "filenames"
   ],
   "rules": {
-    "filenames/match-regex": [2, "^[a-z_]+$"]
+    "filenames/match-regex": [2, "^[a-z0-9_]+(.js)?$"]
   },
   "globals": {
     "$": false,

app/assets/javascripts/application.js

@@ -13,6 +13,7 @@
 /*= require jquery-ui/sortable */
 /*= require jquery_ujs */
 /*= require jquery.endless-scroll */
+/*= require jquery.timeago */
 /*= require jquery.highlight */
 /*= require jquery.waitforimages */
 /*= require jquery.atwho */

changelogs/unreleased/fix-invalid-filename-eslint.yml

+---
+title: Fix invalid filename validation on eslint
+merge_request: 7281
+author: 

spec/javascripts/merge_request_widget_spec.js

 /* eslint-disable */
 /*= require merge_request_widget */
-/*= require lib/utils/jquery.timeago.js */
+/*= require jquery.timeago.js */
 
 (function() {
   describe('MergeRequestWidget', function() {