Add constant for disabling Action Scheduler loading

[m1r0]

Resolves #7135

Proposed Changes

• Introduce a constant (SENSEI_DO_NOT_LOAD_ACTION_SCHEDULER) for disabling the loading of the Action Scheduler library. It's the same one we use in sensei-pro.
• Don't initialize the Sensei Action_Scheduler class and the Migrations if the Action Scheduler library is not loaded.

Testing Instructions

1. Add define( 'SENSEI_DO_NOT_LOAD_ACTION_SCHEDULER', true ); to your wp-config.php
2. Make sure the action scheduler is not loading.

I wasn't able to find a way to add unit tests for this.

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 #7148 (5453444) into trunk (eb0bfa3) will decrease coverage by 0.01%.
Report is 1 commits behind head on trunk.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##              trunk    #7148      +/-   ##
============================================
- Coverage     49.68%   49.68%   -0.01%     
- Complexity    10623    10625       +2     
============================================
  Files           580      580              
  Lines         44876    44877       +1     
  Branches        402      402              
============================================
  Hits          22297    22297              
- Misses        22252    22253       +1     
  Partials        327      327              

Files Changed	Coverage Δ
includes/class-sensei.php	22.93% <0.00%> (-0.06%)	⬇️

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[merkushin]

Works as expected 👍