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/Enums/EbookSortType.php

@@ -6,4 +6,5 @@ enum EbookSortType: string{
 	case AuthorAlpha = 'author-alpha';
 	case ReadingEase = 'reading-ease';
 	case Length = 'length';
+	case Relevance = 'relevance';
 }