Add XML Lint to check the WordPress exports

Problem/Motivation

XML Lint can detect if the wordpress export files are too corrupted or otherwise cannot be processed. This prevents other errors.

Steps to reproduce

XML Lint on the command line shows problems in files. see #3208821: XML (XHR) invalid characters issue

Proposed resolution

Either make it run in Drush or some kind of integrity check at import time.

Add this via composer:

• https://packagist.org/packages/sclable/xml-lint

Then use it via drush or web interface.

Remaining tasks

TBD

User interface changes

TBD

API changes

TBD

Data model changes

Maybe a flag or notice.

(Note I am not actively working on making a patch for this right now but if anyone else wants to we can include it).