Add 6 column grid widget for photo galleries section

[joshdarby]

Changes

This pull request makes the following changes:

• Creates the Borderzine_6_Col_Widget widget (copied from Largo recent posts) to display posts in rows of 6 for the photo galleries section

Why

For #36

Testing/Questions

Features that this PR affects:

• Special series template

Questions that need to be answered before merging:

• [x] Is this PR targeting the correct branch in this repository?

Steps to test this PR:

1. Add the Borderzine 6 Column Widget widget to the Series Life on the Borderline: Footer widget area
   • Title: Photo Galleries
   • Avoid Duplicate Posts
   • Large image
   • Excerpt display: none
   • Hide the publish date
   • Limit to category: Multimedia (?)
2. View widget on frontend and verify that it looks ok

Additional information

INN Member/Labs Client requesting: (if applicable)

• [ ] Contributor has read INN's GitHub code of conduct
• [ ] Contributor would like to be mentioned in the release notes as: (fill in this blank)
• [ ] Contributor agrees to the license terms of this repository.

[benlk]

The code changes in f766833 make sense, but I'm presently experiencing difficulties testing them.

I'm not seeing the widget output on my local site, and even with WP_DEBUG to true there's no output upon the page or in the log. It looks like the widget isn't saving?

[joshdarby]

As a note, https://github.com/INN/umbrella-borderzine/pull/75/commits/8aad4742d0fe6b2ce24a77d6644d365eeba9a2fc sets the widget $context to set podcast => false so no play icons appear due to https://github.com/INN/umbrella-citylimits/issues/87