lightbox: Allow zooming in with video lightbox.

[lennylin1998]

Implement zoom in with video lightbox.
Working Example:

video_lightbox_zoom.mp4

Key changes:

• Wrap InteractiveViwer around SafeArea widget.
  I changed the layer order in _LightboxPageLayout's child because the loading indicator would get zoomed out of screen if I wrap the InteractiveViwer before Stack. Though some syntax changes, the functionality stays the same, and all previous test pass.
• Add test to check video player zooming in/ out correctly.
  I intentionally start the zoom gesture from the corner instead of the center because the wrong setting would leave dead areas where gesutre is not received(see Flutter Doc).
  This is just to make the zoom in functionality for video lightbox coherent with that for image lightbox.

I hope this explain how and why I made those changes. This is my first Zulip PR and open-source contribution. I'd love to hear any suggestions or feedback!

Fixes #1287

[gnprice]

Welcome @lennylin1998, and thanks for the contribution!

This is a post-launch issue, and we're focusing on launch issues until the app is launched. So unfortunately the core team isn't going to be able to spend time reviewing this PR within the next couple of months. I'm marking it as draft to get it out of our review queue until then.

If you'd like to contribute to Zulip, I recommend taking a look at our guide to picking an issue:
https://zulip.readthedocs.io/en/latest/contributing/contributing.html#finding-an-issue-to-work-on