Utility functions and methods

The package's helpers live in appointment/utils/. One page per module:

• date and time operations — date_time.py
• db helpers — db_helpers.py
• email operations — email_ops.py
• error code list — error_codes.py
• ICS calendar files — ics_utils.py
• json and context operations — json_context.py
• permissions handler — permissions.py
• session operations — session.py
• template resolution — template_helpers.py
• validators — validators.py
• general utilities — view_helpers.py