Changes

This pull request makes the following changes:

• Adds a MS Today Single Post widget, which displays a single post by ID.
• Fixes the "circular" call described in https://developer.wordpress.org/reference/functions/is_main_query/#under-the-hood that was affecting a filter on the pre_get_posts action, since I saw that in the debug log:

  [25-Mar-2020 21:00:30 UTC] PHP Notice: is_main_query was called incorrectly. In pre_get_posts, use the WP_Query->is_main_query() method, not the is_main_query() function. See https://codex.wordpress.org/Function_Reference/is_main_query. Please see Debugging in WordPress for more information. (This message was added in version 3.7.0.) in /Users/blk/sites/mstoday/wp-includes/functions.php on line 4986

• Copies in Largo's partials/widget-content.php to fix partials/widget-content.php will never display the byline date WPBuddy/largo#1855 so that the byline date can be unhidden

Widget admin:

Comparison of styles of the Single Post widget and Largo Recent Posts:

Why

For #72, and to make sure that they can enable the date on posts using this code.

Testing/Questions

Features that this PR affects:

• The pre_get_posts fix affects the categories around-the-state and mississippi-roundup. These two categories, and no others, should not display the featured posts section at the top of the category archive
• All Largo Recent Posts widgets, and anything else that uses partials/widget-content.php

Questions that need to be answered before merging:

• Is this PR targeting the correct branch in this repository?
• Do we need a better UI than inserting a raw post ID?
• Does there need to be better descriptive language regarding the post ID?
• Do we need to remove the btn btn-primary classes from the optional "more" link?
• Which widget areas does this need to be tested in?

Steps to test this PR:

0. Check out this branch.
1. Insert a MS Today Single Post widget in a widget area somewhere.
2. Feed it a post ID and save the widget.
3. Verify that the widget displays.
4. Verify that all the widget options work.
5. Switch back to master and review the various Largo Recent Posts widgets on the site. Switch back to this branch. The presence or absence of the date in bylines should be unchanged.