Web: set fixed height for the task table

It looks rather ugly, and this should be addressed, but without the
`height` option of Tabulator it won't be using the Virtual DOM and result
in very slow browser performance.

web/app/src/components/TasksTable.vue

@@ -54,6 +54,7 @@ export default {
       initialSort: [
         { column: "updated", dir: "desc" },
       ],
+      height: "300px",
       data: [], // Will be filled via a Flamenco API request.
       selectable: false, // The active task is tracked by click events.
     };