feat(tasks): make done at column available for selection in table view

This change adds the done at column of tasks to the table view. It also ensures it is possible to sort the tasks by that column.

https://community.vikunja.io/t/is-it-possible-to-list-done-tasks-with-their-completion-date/1922

frontend/src/composables/useTaskList.ts

@@ -21,6 +21,7 @@ export interface SortBy {
 	percent_done?: Order
 	created?: Order
 	updated?: Order
+	done_at?: Order,
 }
 
 // FIXME: merge with DEFAULT_PARAMS in filters.vue