Plugin Directory: Add common-plugin rejection reason for duplicate/common plugin submissions

[davidperezgar]

This PR adds a new rejection reason, common-plugin, to better classify plugin submissions that repeat functionality already widely available in the directory.

What changed

1. Added a new rejection reason key/value in Template::get_rejection_reasons():
   • Key: common-plugin
   • Label: Common plugin
2. Added the corresponding rejection email reason handler for common-plugin in Plugin_Rejected so the correct message is sent when selected.
3. Ensured the new reason is available in the existing rejection reason dropdowns and stored via _rejection_reason like other reasons.

Why
We currently group these cases under broader reasons (often other or duplicate), which makes reporting and reviewer workflows less precise.
common-plugin gives us a clearer, explicit classification for submissions that are redundant in the directory.

Testing

1. Open a plugin in wp-admin and change status to rejected.
2. Confirm Common plugin appears in the rejection reason dropdown.
3. Save and verify _rejection_reason is stored as common-plugin in post meta.
4. Trigger/send the rejection email and confirm the new reason-specific content is used (not fallback other).
5. Verify existing rejection reasons continue to work unchanged.

Notes

• Backward compatible: existing rejection reason keys and flows are unchanged.
• This only introduces a new option and reason template; no migration required.

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