Add account self-deletion

[bpepple]

Summary

• Adds a "Delete Account" button to the user profile page (visible only to the account owner)
• Clicking it leads to a confirmation page that warns the user what data will be lost
• On confirmation, the account is deleted, the session is terminated, and the user is redirected to the home page
• Database contributions (issues, creators, etc.) are preserved; personal data (collection, reading lists) is removed via cascade