Add permission for configuration page

Problem/Motivation

In some cases, the access administration pages permission isn't granular enough, and displays the Webpack configuration page in the menu of some users that don't need to access it. This is usually fine, and makes sense to keep permissions simple, but offering the flexibility would be good.

Steps to reproduce

Add the access administration pages to a user role and observe the Webpack configuration menu item on the configuration page.

Proposed resolution

Add a permission Administer Webpack to allow granular permission assignment.