NotificationBlock: Refactor to mirror Postblock

[marcustyphoon]

Description

We've gone back and forth over this, haven't we?

This switches NotificationBlock's method of selecting which elements are hidden from a CSS selector containing all of the relevant post (root) ids to javascript logic that determines whether to apply a single attribute. This:

• makes it match with the current PostBlock code (we had a PR changing it, but did not go through with it). Less methodological variety means simpler maintenance even if code isn't being directly shared.
• makes it easier to add complex special case logic; I am working on a fix for Mutual Checker, NotificationBlock, Quote Replies: Some notifications are not processed #1658 if the underlying API changes referenced there stay the way they are, and while the same techniques can ultimately be applied to both methods, it's cleaner using this one. edit: on further investigation this probably isn't necessary
• makes it easier to add more features, if desired; this code is actually pulled from NotificationBlock: Add optional placeholders for hidden notifications #1586.

This may wind up as just a commit in another PR. In general, there's a tension between the order of things that minimizes merge conflicts and that which reduces review, probably, but I'm imho really good at managing merge conflicts at this point so ultimately makes no difference.

Testing steps

Confirm that NotificationBlock continues to function as normal, including updates to the shown/hidden state of onscreen notifications when blocking/unblocking (e.g. from the blog view modal on the activity page).