Allow zooming in on videos in the lightbox

From [beta feedback](https://chat.zulip.org/#narrow/channel/48-mobile/topic/Zooming.20videos.20with.20gesture/near/2040646):

> Please make it possible to zoom-in videos played in lightbox. At this moment beta app not allows me to do that. At the same time mobile browser offers such feature to enlarge video with a gesture.

