Feature: Add setting to display course short name in header, resolves #1010

[netanelklein]

This pull request implements a new theme setting that allows administrators to choose whether course headers display the full course name (default behavior) or the course short name.

Background & Use Case

This feature addresses scenarios where course data comes from external systems that may populate the "short name" field with content that's more appropriate for header display than the full course name. While the field is called "short name," external systems sometimes use it for display-friendly course identifiers that may actually be more suitable for headers than the official full course name.

Changes Made

1. Theme Setting (settings.php)
   Added new setting useshortnameinheader under the Course tab in theme configuration
   Uses Yes/No dropdown with "No" as default to maintain backward compatibility
   Setting is logically placed in the "Course header" section alongside related course header options
2. Core Renderer Logic (classes/output/core_renderer.php)
   Modified the full_header() method to check the theme setting
   When enabled and on a course page (excluding site home), the page heading is set to the formatted course short name
   Preserves all existing functionality and header features
   Uses proper context formatting for security and consistency
3. Language Strings (lang/en/theme_boost_union.php)
   Added useshortnameinheader and useshortnameinheader_desc strings
   Strings follow the theme's existing naming conventions and are descriptive for administrators