COVID series landing page updates

[joshdarby]

Changes

This pull request makes the following changes:

• Updates the COVID landing page changes made in Covid series landing page #116
  • Adds "Read more" button to all posts on homepage, author pages, widgets, etc.
  • Styles that new read more button
  • Updates grid for new series child landing page
  • A few other misc. style updates for the new series child landing page

Why

For the COVID series landing page

Testing/Questions

Features that this PR affects:

• COVID series landing page
• Posts being displayed by widget-content.php, archive-category-primary-feature.php, and home-top.php partials

Questions that need to be answered before merging:

• Is this PR targeting the correct branch in this repository?

Steps to test this PR:

1. View all types of pages and widgets to make sure the new read more button works as expected
2. View the COVID landing page and make sure its styling 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.