Ensure the content filter for course content is re-added in Learning Mode

[alexsanford]

Fixes https://github.com/Automattic/evergreen/issues/148

Changes proposed in this Pull Request

When rendering course content in learning mode, ensure the the_content filter is re-added after being temporarily removed. Otherwise, if the_content is called early, the filter is removed and some content may not show up on the frontend.

Testing instructions

One way to see this is by following the steps below. This is an incompatibility with a specific plugin, but it's possible that this could be showing up in other places as well.

• Install the "Rank Math SEO" plugin.
• Create a course with a lesson and a quiz. Enable Learning Mode.
• In Sensei Settings > Courses, ensure the setting "Enable theme styles" is checked and saved.
• Enrol in the course and view the quiz. Without the change from this PR, the quiz questions do not display (Rank Math SEO does something earlier which calls the_content). With this change, the quiz is rendered properly.

[renatho]

I couldn't reproduce the original issue with Rank Math SEO activated, but the change looks good to me, and it continues working to me.

I'd just suggest changing the target branch to feature/post-code-freeze because of the code freeze.

[alexsanford]

Base is updated 👍

We've also tested and confirmed that this fixes the problem on the sites where we were originally seeing this bug.

[renatho]

🎉