moves tz to mixin. applies mixin to views

[mattash]

Fixes #96

This pull request includes changes to enhance timezone handling in views and improve the filtering logic for push notifications in the notifications/utils.py file. The most important changes include adding a new TimezoneMixin and updating the views to use this mixin, as well as refining the filtering of push notifications based on user preferences.

Enhancements to timezone handling:

• hub/views/fast.py: Added TimezoneMixin to the FastListView, FastDetailView, and FastByDateView classes to provide timezone context for serializers. Removed redundant methods for getting timezone from these views. [1] [2] [3] [4] [5] [6] [7]
• hub/views/mixins.py: Introduced the TimezoneMixin class to handle timezone context for serializers, expecting a 'tz' query parameter in IANA format. [1] [2]

Improvements to push notification filtering:

• notifications/utils.py: Enhanced the filtering logic for push notifications to consider user preferences more accurately. Added checks for notification_type and refined exclusion conditions for weekly fasts.

[mattash]

@andylitalo sorry brother. Had to push this one through. Needed to show someone the app.