New Comments Indicator

[EricBAndrews]

Add an optional indicator to the post view to tell the user if the post has new comments since they last viewed it.

Backend:
It looks like the API has something approaching support for this--read_post performs a count of the comments at the time of read. If we can get that information when we fetch a post, we could use in the obvious manner to determine if there are new comments.

Frontend:
Replace the standard comment indicator with text.bubble when there are new replies

Ideally we could then highlight the comments themselves. It may be necessary to perform client-side caching of read comments, which shouldn't be too expensive--all we'd need to store is the id, which is only a few bytes.

[Sjmarf]

PostView has an unread_comments integer value that we could use to do this. It won't tell us which comments are new, but it'll tell us that they exist. https://join-lemmy.org/api/interfaces/PostView.html#unread_comments

[Sjmarf]

Added in #858