feat(caldav): Add support for subtasks (i.e. RELATED-TO property) in CalDAV (#1634)

As I mentioned [here](https://kolaente.dev/vikunja/api/pulls/1442#issuecomment-55215), this is mainly a cleanup of @zewaren 's original [PR](https://kolaente.dev/vikunja/api/pulls/1442).

It adds support for the `RELATED-TO` property in CalDAV's `VTODO` and the `RELTYPE=PARENT` and `RELTYPE=CHILD` relationships. In other words, it allows for `ParentTask->SubTask` relations to be handled supported through CalDAV.

In addition to the included tests, this has been tested by both @zewaren & myself with DAVx5 & Tasks (Android) and it's been working great.

Resolves https://kolaente.dev/vikunja/api/issues/1345

Co-authored-by: Miguel A. Arroyo <miguel@codeheads.dev>
Co-authored-by: Erwan Martin <public@fzwte.net>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1634
Reviewed-by: konrad <k@knt.li>
Co-authored-by: Miguel Arroyo <mayanez@noreply.kolaente.de>
Co-committed-by: Miguel Arroyo <mayanez@noreply.kolaente.de>

pkg/caldav/caldav_test.go

@@ -326,6 +326,49 @@ ACTION:DISPLAY
 DESCRIPTION:Todo #1
 END:VALARM
 END:VTODO
+END:VCALENDAR`,
+		},
+		{
+			name: "with related-to",
+			args: args{
+				config: &Config{
+					Name:   "test",
+					ProdID: "RandomProdID which is not random",
+				},
+				todos: []*Todo{
+					{
+						Summary:     "Todo #1",
+						Description: "Lorem Ipsum",
+						UID:         "randommduid",
+						Relations: []Relation{
+							{
+								Type: models.RelationKindParenttask,
+								UID:  "parentuid",
+							},
+							{
+								Type: models.RelationKindSubtask,
+								UID:  "subtaskuid",
+							},
+						},
+						Timestamp: time.Unix(1543626724, 0).In(config.GetTimeZone()),
+					},
+				},
+			},
+			wantCaldavtasks: `BEGIN:VCALENDAR
+VERSION:2.0
+METHOD:PUBLISH
+X-PUBLISHED-TTL:PT4H
+X-WR-CALNAME:test
+PRODID:-//RandomProdID which is not random//EN
+BEGIN:VTODO
+UID:randommduid
+DTSTAMP:20181201T011204Z
+SUMMARY:Todo #1
+DESCRIPTION:Lorem Ipsum
+LAST-MODIFIED:00010101T000000Z
+RELATED-TO;RELTYPE=PARENT:parentuid
+RELATED-TO;RELTYPE=CHILD:subtaskuid
+END:VTODO
 END:VCALENDAR`,
 		},
 	}