Add a Relevance sort order and improve search

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`);
```

lib/Exceptions/EbookIndexableAuthorsRequiredException.php

@@ -0,0 +1,7 @@
+<?
+namespace Exceptions;
+
+class EbookIndexableAuthorsRequiredException extends AppException{
+	/** @var string $message */
+	protected $message = 'Ebook IndexableAuthors required.';
+}