Feature: Add bulk booking option for students

[Smartlearn-edu]

Summary

Add a per-activity setting that allows students to book all available slots at once with a single click, instead of booking them individually.

Changes

New activity setting

• Added "Enable bulk booking" (Yes/No) option under the activity settings form
• When enabled, the appointment mode (booking limit) settings are automatically disabled

Student interface

• When bulk booking is enabled, a prominent "Book all available slots" button is displayed at the top of the available slots list
• Individual "Book slot" buttons are hidden to guide students toward the bulk action
• The slot table remains visible so students can review available slots before booking

Backend

• New bulkbook field added to the scheduler database table
• count_bookable_appointments() returns unlimited (-1) when bulk booking is active
• New bookallslots controller action that iterates all available slots, creates appointments directly, triggers booking events, and sends teacher notifications
• Proper validation: checks slot capacity, bookable period, and prevents duplicate bookings

Database

• Schema updated in install.xml and upgrade.php (version 2026040300)

Language strings

• bulkbook, bulkbook_help, bookallslots, bulkbook_message

Technical note

Avoided using PHP's empty() on model properties because the plugin's mvc_record_model class uses __get() magic without implementing __isset(), which causes empty() to always return true. Used direct property access with (bool) casting instead.