Fix padding in site editor that prevents full-width alignment from working

[donnapep]

Resolves #7227.

Proposed Changes

Removes the padding from wp-site-blocks and adds it to sensei-course-theme__main-content instead.

We need to be careful going forward not to add CSS that could affect other block templates, which means we need to target selectors that are only available in Sensei's own block templates.

Testing Instructions

1. Download the Annalee theme from pNEWy-hkA-p2 and activate it.
2. In the site editor, edit the "Front Page" template.
3. Scroll down to the "Ready to transform your life?" group.
4. Confirm that the group block's alignment is set to "Full width" and that the block extends to the left and right edges of the editor.
5. As a student, view a Learning Mode lesson in mobile view.
6. Ensure that the content does not touch the edges of the browser.

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
• 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

[codecov]

Codecov Report

Merging #7233 (c28c918) into trunk (79c3a7e) will not change coverage.
Report is 1 commits behind head on trunk.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              trunk    #7233   +/-   ##
=========================================
  Coverage     50.24%   50.24%           
  Complexity    10862    10862           
=========================================
  Files           601      601           
  Lines         45753    45753           
  Branches        402      402           
=========================================
  Hits          22989    22989           
  Misses        22437    22437           
  Partials        327      327           

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[Imran92]

Works nicely 👍 Thanks for working on the Annalee theme issues