Call stuff "topics" and "posts" instead of "posts" and "comments"

[tanius]

The Graphryder software should use the same nomenclature throughout that is also used in Discourse, because it will not be used outside of Discourse for the foreseeable future. This is not yet the case, at least not consequently. For example, see this output of http://localhost:5000/status:

{
  "labels": [
    "Users",
    "Posts",
    "Comments",
    "Annotations",
    "Tags"
  ],
  "data": [ …]
}