Make the anonymous account id = -1

This change also simplifies the site index query.

src/sql/insert_anon_author.sql

@@ -1,10 +1,12 @@
 INSERT OR IGNORE INTO authors (
+	id,
 	name,
 	salt,
 	passhash,
 	joindate,
 	biography
 ) VALUES (
+	-1,
 	'anonymous',
 	'',
 	'',

src/sql/select_site_index.sql

@@ -9,17 +9,6 @@ FROM
 	authors
 WHERE
 	posts.authorid = authors.id
-UNION
-SELECT
-	posts.id,
-	posts.post_title,
-	posts.isanon,
-	posts.post_time,
-	'Anonymous'
-FROM
-	posts
-WHERE
-	posts.authorid = -1
 ORDER BY
 	posts.post_time DESC
 LIMIT 10;