feat(caldav): Add support for subtasks in CalDAV.

When exporting tasks with CalDAV, export their relationship to their
parent tasks.
When creating/updating tasks from CalDAV, create/maintain their parent
relationship using task relations.

@@ -523,0 +536,4 @@
+						UID:                "randommduid",
+						Timestamp:          time.Unix(1543626724, 0).In(config.GetTimeZone()),
+						Color:              "affffe",
+						RelatedToParentUID: "another_random_uid",

@@ -42,0 +42,4 @@
+		// Find the UID of the parent task, if it exists:
+		var parentTaskUID string
+		if parentTasks, ok := t.RelatedTasks[models.RelationKindParenttask]; ok {
+			parentTaskUID = parentTasks[0].UID

@@ -60,3 +60,3 @@
 
 // InitTests handles the actual bootstrapping of the test env
-func InitTests() {
+func InitTests(loadFixtures bool) {

@@ -72,3 +72,1 @@
-	err = db.LoadFixtures()
-	if err != nil {
-		return
+	if loadFixtures {

@@ -433,2 +451,4 @@
 }
 
+// When a VTODO entry doesn't have a parent anymore, but we do, we need to remove it as well.
+func removeLegacyParentRelations(s *xorm.Session, a web.Auth, task *models.Task, newRelations map[models.RelationKind][]*models.Task) (err error) {