Fix incorrect URLs for edited emails

[donnapep]

Resolves #7621.

Proposed Changes

Strips out the URL protocol if necessary when replacing URL placeholders in the email content (e.g. href="http://[completed:url]" or href="https://[completed:url]"). The placeholder value already contains the protocol and it was being duplicated, resulting in the issue.

Note:

• I haven't included any test coverage in order to ship this change quickly in today's release.
• The proper solution would be to not prepend the protocol on save, but that solution is more involved so I opted for a faster one.

Testing Instructions

1. Open the Student Course Completed email.
2. Select the View Results button and click the pencil icon to edit it. The URL should be [completed:url].
3. Click Open in a new tab.
4. Save the change and the email.
5. Edit the button again and see that the URL is now http://[completed:url]. (This PR does not address this issue.)
6. As a student, complete a course.
7. Click the View Results button and ensure the link is correct.
8. Click Recreate Emails in Sensei > Tools.
9. Double check that the URL for the button has been reset to [completed:url]. Don't make any changes or save the email.
10. As a student, complete a course.
11. Click the View Results button and ensure the link is correct.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Legacy courses (course without blocks) are tested
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[Imran92]

Works nicely as expected! Thanks for fixing this!