feat(caldav): Export Labels to Caldav (#1409)

Partially resolves https://kolaente.dev/vikunja/api/issues/1274

Co-authored-by: ce72 <christoph.ernst72@googlemail.com>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1409
Reviewed-by: konrad <k@knt.li>
Co-authored-by: cernst <ce72@noreply.kolaente.de>
Co-committed-by: cernst <ce72@noreply.kolaente.de>

pkg/caldav/caldav_test.go

@@ -481,6 +481,45 @@ DUE:20181201T011204Z
 RRULE:FREQ=SECONDLY;INTERVAL=435
 LAST-MODIFIED:00010101T000000Z
 END:VTODO
+END:VCALENDAR`,
+		},
+		{
+			name: "with categories",
+			args: args{
+				config: &Config{
+					Name:   "test",
+					ProdID: "RandomProdID which is not random",
+					Color:  "ffffff",
+				},
+				todos: []*Todo{
+					{
+						Summary:    "Todo #1",
+						UID:        "randommduid",
+						Timestamp:  time.Unix(1543626724, 0).In(config.GetTimeZone()),
+						Color:      "affffe",
+						Categories: []string{"label1", "label2"},
+					},
+				},
+			},
+			wantCaldavtasks: `BEGIN:VCALENDAR
+VERSION:2.0
+METHOD:PUBLISH
+X-PUBLISHED-TTL:PT4H
+X-WR-CALNAME:test
+PRODID:-//RandomProdID which is not random//EN
+X-APPLE-CALENDAR-COLOR:#ffffffFF
+X-OUTLOOK-COLOR:#ffffffFF
+X-FUNAMBOL-COLOR:#ffffffFF
+BEGIN:VTODO
+UID:randommduid
+DTSTAMP:20181201T011204Z
+SUMMARY:Todo #1
+X-APPLE-CALENDAR-COLOR:#affffeFF
+X-OUTLOOK-COLOR:#affffeFF
+X-FUNAMBOL-COLOR:#affffeFF
+CATEGORIES:label1,label2
+LAST-MODIFIED:00010101T000000Z
+END:VTODO
 END:VCALENDAR`,
 		},
 	}