Rename Things.PostContainer to Things.PostsContainer and Things.CommentContainer to Things.CommentsContainer

[sirkris]

Just realized these are confusingly named. The name suggests that it's a container for a single post/comment, when in fact it contains a list of posts/comments. That list will often be populated with a single entry, but still. This change will make it clearer that these containers can hold multiple post/comment objects.

Because this will likely lead to BC breakage in userland, the original names will remain in deprecated form for the time being.