Allow Post component to render hidden thread placeholder as needed

[virtuous-cat]

(Ms boba has edited this)

On this updated design, HiddenThread should wrap the original Post as a child, like this:

<HiddenThread /* props */>
  <Post /* props */ />
</HiddenThread>

By default, HiddenThread should not display Post unless its hidden prop equals false. Then HiddenThread should wrap Post with additional UI (see attachment).

One (slight) difficulty is that HiddenThread should avoid displaying the wrapped Post's content notices since they will already be displayed by HiddenThread. This can be done by cloning the Post child and passing a new tags prop that has no content notices.

The buttons at the top should follow down the post as the user scrolls.