Hardcode the Digest Sidebar, style fixes for certain widgets therein

[benlk]

Changes

This pull request makes the following changes:

• Modifies ENN's sidebar.php to load a custom sidebar partial on the digest category page
• Adds a sidebar template to output the newly-created Digest Sidebar, or, if that is empty, nothing
• Registers a Digest Sidebar with appropriate description of where it is employed.

Why

For #86

Testing/Questions

Features that this PR affects:

• /category/digest/

Questions that need to be answered before merging:

• Is this PR targeting the correct branch in this repository?
• do we want to do any specific styling for textwidget, with padding or anything like that? See Create digest page from Wireframe #86 (comment)

Steps to test this PR:

1. Check out this branch
2. In Appearance > Widgets, add some widgets to the Digest Sidebar widget area. I added:
   • a text widget identifying the sidebar
   • a ENN Mailchimp signup widget
   • a textwidget that contains the following:

     <p>
     	Are you subscribed to one or more daily digests? Add or remove subscriptions by following this link:
     </p>
     
     <a class="btn btn-primary" href="https://example.org">Manage Subscription</a>

3. Visit the following locations:
   • /category/digest/ : the Digest sidebar should be displayed
   • /category/news/ : the Digest sidebar should not be displayed
   • /region/west/ : the Digest sidebar should not be displayed
   • /category/digest/ with a search query: the Digest sidebar should be displayed

[joshdarby]

@benlk Should we switch the target branch to staging instead of master?

[benlk]

See updated widget screenshots in #86.

[benlk]

This is now deployed, but not yet configured with widgets.

[joshdarby]

I don't actually see this deployed on staging 😕

And I don't have access to their WPE account to be able to deploy it.

[benlk]

Deployed for real now. Proceeding to config.

[benlk]

Configured: https://usenergystage.wpengine.com/category/digest/