Display a reason for each notification

[rossjrw]

#85 added a set of flags that indicate which context/contexts is/are responsible for a notification being raised:

notifier/notifier/database/queries/get_notifiable_posts_for_user.sql

Lines 26 to 42 in e7dc8cd

	-- Flags indicating the reasons that a post emits a notification
	CASE WHEN (
	thread_sub.sub = 1
	) THEN 1 ELSE 0 END AS flag_user_subscribed_to_thread,
	CASE WHEN (
	post_sub.sub = 1
	) THEN 1 ELSE 0 END AS flag_user_subscribed_to_post,
	CASE WHEN (
	context_thread.first_post_author_user_id = %(user_id)s
	) THEN 1 ELSE 0 END AS flag_user_started_thread,
	CASE WHEN (
	context_parent_post.author_user_id = %(user_id)s
	) THEN 1 ELSE 0 END AS flag_user_posted_parent

These should be used in the digest to justify each notifcation's existence, which prompts the user to deduplicate contexts and also lets them know exactly how to e.g. unsubscribe from a given notification specifically. Will be increasingly important as new contexts are added (#77, #86, etc).