Add feature for email functionality

[joshdarby]

Changes

This pull request makes the following changes:

• Added a Email Article button in the republish modal, next to Copy to Clipboard, to allow users to email the article content and pixel in this format:

 * Subject:
 * %2$s - "Republish this story:"
 * %3$s - post permalink
 * 
 * Body:
 * %5$s - $article_info The article title, byline, source site, and date
 * %0D%0A - carriage return, line feed
 * %6$s - $content The article content
 * %0D%0A - carriage return, line feed
 * %7$s - $attribution_statement The article source
 * %0D%0A - carriage return, line feed
 * %8$s - $pixel The link for the republication tracking pixel

• Sets $hide_republication_widget_by_filter = false; as a default before applying the hide_republication_widget filter, because of "Hide Republish Button" checkbox on posts shows "The Republication sharing widget on this post is programatically disabled..." message for filter only #67

Why

For #68 (not sure why I named this branch 27-* 🤦‍♂), #67

Testing/Questions

Features that this PR affects:

• Shareable content modal + email functionality
• Hide Republication Widget metabox on post editor

Questions that need to be answered before merging:

• Is this PR targeting the correct branch in this repository?
• Does the escaping/formatting look ok for the email subject and body? It's mostly copied from how largo_post_social_links formats the email sharing link

Steps to test this PR:

Email functionality:

1. Go to a post with the widget
2. Open modal and click Email Article
3. Verify the formatting matches what's posted above
4. Verify the content looks sane and the pixel and whatnot are inserted as expected.

Hide Republication Widget metabox:

1. Open the post editor for a post
2. Check Hide the Republication sharing widget on this post? to be true
3. Save
4. On refresh, make sure you can uncheck the checkbox instead of seeing the The Republication sharing widget on this post is programatically disabled through the hide_republication_widget filter. warning.

[adekbadek]

Hi @joshdarby – could you please resolve the conflicts here?

[benlk]

@adekbadek You or another Automattician will need to resolve conflicts on this one.