Fix Sensei Email template not displayed in site editor

[donnapep]

Resolves #7460.

Proposed Changes

Revert 198145c and add an additional check to return early and not add the email template if we're not in the site editor.

Testing Instructions

• Go to Appearance > Editor > Templates.
• Look for Sensei Email in the list of templates.
• Click to edit the template and ensure you're able to.
• Go to WP Admin > Posts > Tags.
• Add a new tag and click on View. The tag should remain empty.
• You shouldn't see the email template, but an empty tag listing.
• Go to Sensei LMS > Settings > Emails and ensure you are able to preview and edit emails.

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

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (9f25df0) 51.21% compared to head (85fdd5f) 51.20%.
Report is 1 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk    #7463      +/-   ##
============================================
- Coverage     51.21%   51.20%   -0.01%     
- Complexity    11188    11189       +1     
============================================
  Files           614      614              
  Lines         47228    47230       +2     
  Branches        405      405              
============================================
  Hits          24186    24186              
- Misses        22715    22717       +2     
  Partials        327      327              

Files	Coverage Δ
...udes/internal/emails/class-email-page-template.php	70.00% <77.77%> (-5.00%)	⬇️

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[Imran92]

Looks good and works as expected