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.
This allows the user to run a keyword search and then change the sort
order. `Default` is interpreted as `Relevance` if a query is present,
`Newest` if not.
Here's what's in `IndexableText` right now:
1. Title
2. Collections
3. Authors
4. Tags
5. LocSubjects
6. TocEntries
Here is the proposed new ranking:
```
10 * Title +
8 * Authors +
3 * Collections +
IndexableText
```
New indices for existing DBs:
```
ALTER TABLE `Ebooks` ADD COLUMN `IndexableAuthors` text NOT NULL;
ALTER TABLE `Ebooks` ADD COLUMN `IndexableCollections` text NULL;
ALTER TABLE `Ebooks` ADD FULLTEXT `indexSearchTitle` (`Title`);
ALTER TABLE `Ebooks` ADD FULLTEXT `idxSearchAuthors` (`IndexableAuthors`);
ALTER TABLE `Ebooks` ADD FULLTEXT `idxSearchCollections` (`IndexableCollections`);
```
