This commit adds a rewrite rule for ebook downloads of the form:
```
/ebooks/some-author/some-book/downloads/some-filename.epub
```
to `www/ebooks/download.php`. That file handles the logic of whether to
show a thank you page before beginning the download.
Download URLs in RSS, Atom, and OPDS feeds follow the same pattern, but
they have a query string parameter `?source=feed` to always skip the
thank you page.
* Replace `EbookUrl` with `EbookId` in `Artworks`
* Add a `FullUrl` member to `Ebook`
Add documentation about when to use it versus Url.
The full URL is also being used as an ID in RSS feeds, so use `FullUrl` there.
* Store an `EbookId` in `Artworks`
