[ST-2529] Notifications bugs - #3174

Three bugs fixed:

1. Notifications task was running every 72 hours but only looking 24 hours ahead/back, so some phase start/end and upcoming event notifications were getting missed
2. User setting for newsletter opt-in was getting cleared out by missing field on dashboard form
3. After guest user convert to regular user, notifications were not getting sent because user was still being identified as guest due to guest user row not being deleted

Tasks

• PR name contains story or task reference
• Documentation (docs and inline)
• Tests (including n+1 and django_assert_num_queries where applicable)
• Changelog

:.::..:::::: Aligned Asana Task ::.:.::.::::