[2.0] Rewrite ExpandedPostView

[Sjmarf]

I think we should fully rewrite the comment system. In my opinion, we should:

• Improve the collapse animation to make it less jarring when collapsing a large comment stack. This might be made easier by defining the comment views recursively, such that each comment view renders all of its children.
• Add a "show more replies" button akin to what lemmy-ui has for very large posts where not all comments are initially loaded. Currently, we don't attempt to load the additional comments at all (Child comments in some posts don't load #844). The button should either add the extra comments to the view, or push a new view onto the navigation stack that displays the new comments. The first option is more convenient, but the second may be necessary if the comment depth gets so great that we can't fit the comments on the screen anymore. Potentially, we could support both options and choose which to use based or the number of replies and/or the depth of those replies.
• Fix our "Hot" sorting and add support for "Controversial" ("hot" comment sort mode consistency with Lemmy #804)