Add description field to feed settings

This adds a new "description" field to the feed settings. This allows to
save custom description regarding a feed. It is also exported and
imported as "description" in OPML.

internal/database/migrations.go

@@ -898,4 +898,9 @@ var migrations = []func(tx *sql.Tx) error{
 		_, err = tx.Exec(sql)
 		return err
 	},
+	func(tx *sql.Tx) (err error) {
+		sql := `ALTER TABLE feeds ADD COLUMN description text default ''`
+		_, err = tx.Exec(sql)
+		return err
+	},
 }