Use mixing for relative date in card sidebar

Signed-off-by: Julius Härtl <jus@bitgrid.net>

src/store/card.js

@@ -147,6 +147,7 @@ export default {
 			if (existingIndex !== -1) {
 				Vue.set(state.cards[existingIndex], property, card[property])
 			}
+			Vue.set(state.cards[existingIndex], 'lastModified', Date.now() / 1000)
 		},
 		cardIncreaseAttachmentCount(state, cardId) {
 			const existingIndex = state.cards.findIndex(_card => _card.id === cardId)