Adding Date Modified sorting for Articles Manager in backend

Pull Request for Issue #15610

Summary of Changes

4 files modified

1. Including Date Modified field in the model
   administrator/components/com_content/models/articles.php

2. Adding the option in the ordering list form field with language constant.
   administrator/components/com_content/models/forms/filter_articles.xml

3. Modifying the tmpl file, to consider the new date ordering label on the table's headings.
   administrator/components/com_content/views/articles/tmpl/default.php

4. Adding en-GB translations for the new constants
   administrator/language/en-GB/en-GB.com_content.ini
   Totally 3 new constants:
   COM_CONTENT_HEADING_DATE_MODIFIED="Date Modified"
   COM_CONTENT_MODIFIED_ASC="Date Modified ascending"
   COM_CONTENT_MODIFIED_DESC="Date Modified descending"

Testing Instructions

After applying the patch, go in articles manager in backend and through the Sorting List select the Date Modified ASC/DESC options to sort articles by date modified.

Expected result

The articles should be ordered by this date option.

Documentation Changes Required

Mention that articles manager now supports ordering the articles by Date Modified using the Sorting List field.