Add project description field.

Description is shown in a popup when hovering the 'information' icon, like column description,
which is shown in project list on dashboard and in board's title.

This is useful to document the categories and colors used in the project.

app/Schema/Postgres.php

@@ -6,7 +6,12 @@ use PDO;
 use Core\Security;
 use Model\Link;
 
-const VERSION = 27;
+const VERSION = 28;
+
+function version_28($pdo)
+{
+    $pdo->exec('ALTER TABLE projects ADD COLUMN description TEXT');
+}
 
 function version_27($pdo)
 {