feat: Add option of sending emails to feed posts
- Separated tasks into their own files in app.tasks to avoid import errors
- Added task for sending mails in app.tasks.send_mail_to_members.py
- Separated out WebAppOptions from app.web.web_server into app.web.web_app_options to avoid import errors
- Added optional argument "email" to APIPostHandler in app.web.handlers.api.feed.post.py which schedules the task in app.tasks.send_mail_to_members.py
- Small fixes to type hinting etc