Update the course/lesson placeholder to use an internal image

[m1r0]

Resolves https://github.com/Automattic/sensei-security/issues/27

Proposed Changes

• Update the placeholder to use an internal image instead of an external one (via.placeholder.com).

Testing Instructions

1. Go to Sensei LMS -> Settings -> General and enable the "Use placeholder images" option.
2. Go to Sensei LMS -> Settings -> Lessons and enable the "Single Lesson Images" option.
3. Go to the courses archive page and make sure the course placeholder is visible.
4. Go to a single lesson page and make sure the lesson placeholder is visible.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• 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
• Code is tested on the minimum supported PHP and WordPress versions

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[donnapep]

Looks like this hasn't worked for some time as via.placeholder.com isn't a valid site. But these images only seem to display when Learning Mode is disabled and the course archive page isn't using blocks, so likely minimal impact.