Extract email template to a template file

[alexsanford]

Closes #85

This PR extracts the rendering of the content drip email to a template file. It uses Sensei's template file loading so that the template may be overridden by the theme.

Testing Instructions

Ensure the email template renders properly

• Create a few courses with some lessons in each one.
• Set the Content Drip setting for the lessons to a "specific date" and set the date to today.
• Manually run the woo_scd_daily_cron_hook cron event (the "WP Crontrol" plugin is one way to do this).
• Ensure that the email is sent properly, and that it looks right. To verify, switch to the master branch, run the event again, and compare the email from the master branch with the email from this branch. They should be identical.
• Switch back to this branch before continuing.
• Reorder some dripped lessons within a course that are not in a module. Ensure that the lesson order within the email matches the lesson order in the course. NOTE: this does not work for lessons within a module, and it never did work before. See Lesson order in drip email is wrong for lessons within modules #154
• Modify some of the email strings within Sensei > Settings > Content Drip. Send the drip email again. Ensure that the modified strings are used.

Ensure template overriding works

• In your theme directory, create a subdirectory sensei-content-drip.
• Copy the templates/single-email-drip-notification.php file into that directory.
• Modify the single-email-drip-notification.php within the theme.
• Run the cron event. Ensure that the modified template was used.

[donnapep]

Couple of notes on escaping URLs. Otherwise 👍

[alexsanford]

Note that I rebased this to fix a conflict

[jom]

Looks good; works well!

[richardmtl]

This is great news!

[regiside]

Hey,
Not sure if anyone here has access, but it would be nice if this info was added to the Sensei Content Drip documentation. https://senseilms.com/documentation/sensei-content-drip/ The docs are very lacking for anyone trying to do any kind of customization.