Posts, like 8661f4724aa9, can go missing if the account or post was
removed. In this case, the API returns data like this:
```json
{
"data": {
"post": null
}
}
```
When this happens, we can detect it because the parsed response now has
a nil value: `response.data.post == nil` and construct an `EmptyPage`
instead of a `Page`. The `Articles::Show` action can then render
conditionally based on if the response from `PageConverter` is a `Page`
or an `EmptyPage`.
The subtitle has been removed because it's difficult to find and error
prone to guess at. It is somewhat accessible from the post's
previewContent field in GraphQL but that can be truncated.
Right now this links to the user's medium page. It may link to an
internal page in the future.
Instead of the Page taking the author as a string, it now takes a
PostResponse::Creator object. The Articles::ShowPage then converts the
Creator (a name and user_id) to an author link.
Finally, I did some refactoring of UserAnchor (which I thought I was
going to use for this) to change it's userId attribute to user_id as is
Crystal convention.
Medium guides each post to have a Title and Subtitle. They are rendered
as the first two paragraphs: H3 and H4 respectively. If they exist, a
new PageConverter class extracts them and sets them on the page.
However, they aren't required. If the first two paragraphs aren't H3
and H4, the PageConverter falls back to using the first paragraph as
the title, and setting the subtitle to blank.
The remaining paragraphs are passed into the ParagraphConverter as
normal.
