Add better support for scrolling content to our modals

[abey79]

What

This PR builds proper scrollability to our modals, thereby fixing:

• an ugly glitch with large add/remove entity modal
• add view/container modal not scrollable with very small viewer window size

Before

After

[github-actions]

Web viewer failed to build.

| Result | Commit | Link | Manifest |
| ------ | ------- | ----- |
| ❌ | | https://rerun.io/viewer/pr/8492 | +nightly +main |

^{Note: This comment is updated whenever you push a commit.}

[teh-cmc]

Can we add a checklist for modals? Even something extremely barebones ("click on add container and make sure it's not insane"), so the release manager are reminded to check them.

[teh-cmc]

I don't understand the code, but it works.

See my comment above: add a checklist please.