Add real buckets for tasks which don't have one (#446)

Add docs for error code

Add moving new tasks into the default bucket when none was provided

Add moving tasks in default bucket when deleting one

Fix tests again

Add test for removing a bucket

Fix tests

Prevent removing the last bucket

Remove the empty pseudo bucket

Add migration to create a new bucket for each list (and put all tasks in it

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/446

pkg/models/tasks_test.go

@@ -47,6 +47,8 @@ func TestTask_Create(t *testing.T) {
 		// Assert getting a new index
 		assert.NotEmpty(t, task.Index)
 		assert.Equal(t, int64(18), task.Index)
+		// Assert moving it into the default bucket
+		assert.Equal(t, int64(1), task.BucketID)
 
 	})
 	t.Run("empty text", func(t *testing.T) {