🎨 Update avatar URL for author notifications

Change the avatar URL in the author joined notification to ensure consistency with branding.


♻️ Refactor application helper and views for author profiles

- Commented out regex expression modification in `application_helper.rb`.
- Added `calculate_age` method to compute age based on birth and death dates.
- Updated `life_in_weeks.html.erb` to display birth and death information with icons and age calculations.
- Enhanced `milestones.html.erb` to include author avatar display.
- Modified `profile.html.erb` to use a consistent avatar style.
- Improved `profiles.html.erb` layout with blurred avatar backgrounds and additional author information.
- Updated `import_milestones.rake` to include death date for authors.
- Added comprehensive tests for `calculate_age` in `application_helper_test.rb`.
- Imported life milestones for Janis Joplin, Jim Morrison, Jimi Hendrix, John Lennon, Kurt Cobain, and Osama bin Laden.


✨ Add life events for multiple historical figures

Include detailed life events for Amy Winehouse, Janis Joplin, Jim Morrison,
Jimi Hendrix, John Lennon, Kurt Cobain, and Osama bin Laden in CSV format.