[feature] Improve UX of the Notifications Module

[pandafy]

Closes #132
Closes #306
Closes #110
Closes #148
Closes #255

[pandafy]

@Dhanus3133 can you ensure that the coverage does not decrease for these files:

• api/serializers.py
• base/models.py
• tasks.py

[Dhanus3133]

@pandafy For the coverage missing on base/models.py

openwisp-notifications/openwisp_notifications/base/models.py

Lines 302 to 304 in b7dff78

	def email_notification(self):
	if self.email is not None:
	return self.email

Seems to be due to the defaults being set here

openwisp-notifications/openwisp_notifications/tasks.py

Lines 88 to 107 in 941982d

	def create_notification_settings(user, organizations, notification_types):
	global_setting, _ = NotificationSetting.objects.get_or_create(
	user=user, organization=None, type=None, defaults={'email': True, 'web': True}
	)
	for type in notification_types:
	notification_config = types.get_notification_configuration(type)
	for org in organizations:
	NotificationSetting.objects.update_or_create(
	defaults={
	'deleted': False,
	'email': global_setting.email
	and notification_config.get('email_notification'),
	'web': global_setting.web
	and notification_config.get('web_notification'),
	},
	user=user,
	type=type,
	organization=org,
	)

But if that's the case

openwisp-notifications/openwisp_notifications/base/models.py

Lines 308 to 310 in b7dff78

	def web_notification(self):
	if self.web is not None:
	return self.web

this should also cause a coverage issue but it doesn't, not sure why.

[nemesifier]

Please resolve conflicts with master and squash commits so we have 1 commit for each feature/change.

I hope we can merge after this.