fix: dont html encode ampersands in subject

[leogermani]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Fixes a small issue where ampersands are HTML encoded in the Subject.

Not sure if there's a better way to do it. But it's weird because the only character that is being HTML encoded is the ampersand, so loading a library to html_decode everything looked unnecessary.

How to test the changes in this Pull Request:

1. In trunk edit a newsletters
2. add a & to the title and see that $amp; is added to the Subject input in the Newsletter sidebar
3. Checkout this branch and do the same
4. See that & is preserved as is

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 20.38%. Comparing base (44c1b12) to head (858cb56).

Files with missing lines	Patch %	Lines
...or/class-newspack-newsletters-campaign-monitor.php	0.00%	2 Missing ⚠️
...ign/class-newspack-newsletters-active-campaign.php	0.00%	1 Missing ⚠️
...ct/class-newspack-newsletters-constant-contact.php	0.00%	1 Missing ⚠️
...mailchimp/class-newspack-newsletters-mailchimp.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk    #1686   +/-   ##
=========================================
  Coverage     20.38%   20.38%           
  Complexity     2664     2664           
=========================================
  Files            48       48           
  Lines         10619    10619           
=========================================
  Hits           2165     2165           
  Misses         8454     8454           

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[dkoo]

@leogermani this didn't work for me for some reason—the ampersands were still coming in as & in the the subject text input when editing the post title field, and when synced to the ESP the subject still contained &. I think WP converts certain characters into HTML entities when the strings are saved into the DB.

Here's what I had to do to fix the issue: #1687

This doesn't try to save the title with unescaped characters, it just displays the unescaped string in the sidebar field, and then decodes the HTML entities in the sync payload that gets sent to the ESP. Let me know what you think!

[leogermani]

@dkoo Your PR looks much more robust! Feel free to take it over!

[dkoo]

@leogermani I've merged #1678 into this PR. Paging @Automattic/newspack-product for another review!