feat: emoji reactions for tasks and comments (#2196)

This PR adds reactions for tasks and comments, similar to what you can do on Gitea, GitHub, Slack and plenty of other tools.

Reviewed-on: https://kolaente.dev/vikunja/vikunja/pulls/2196
Co-authored-by: kolaente <k@knt.li>
Co-committed-by: kolaente <k@knt.li>

pkg/models/task_collection_test.go

@@ -98,6 +98,9 @@ func TestTaskCollection_ReadAll(t *testing.T) {
 		BucketID:    1,
 		IsFavorite:  true,
 		Position:    2,
+		Reactions: ReactionMap{
+			"👋": []*user.User{user1},
+		},
 		Labels: []*Label{
 			label4,
 		},