Add cancel button to media library widget popup

Problem/Motivation

A cancel button is standard on most modals. But it's missing on the media library widget popup.

Steps to reproduce

1. Enable media and media library modules
2. Add a media field to a taxonomy (/admin/structure/taxonomy/manage/tags/overview/fields)
3. Add a term to the vocabulary (/admin/structure/taxonomy/manage/tags/add)
4. Click Add media button
5. See the media library popup, but no cancel button

Proposed resolution

Add a cancel button

User interface changes

There will be a cancel button