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