Window Briefly Flashes White While Launching

## Expected Behavior

When Element Desktop is launching and the window first appears, the window should show the user interface (e.g. a loading screen or similar).

## Actual Behavior

When Element Desktop is launching and the window first appears, the window briefly flashes white before the loading animation is shown.

[I have uploaded a short screen recording of the issue to YouTube.](https://youtu.be/tGlEVuesQjk)

## Steps to reproduce

Launch Element Desktop when it is not already running.

(The white flash may present itself in other situations, as well.)

## Version Information

* Operating system: macOS 11.5.2
* Element version: 1.8.5
* Installed from https://element.io/get-started
* Homeserver: matrix.org
* rageshake not submitted

## Commentary

The white flash when launching an Electron application is a known issue that should be easy to fix. The white flash occurs because the `BrowserWindow` instance is shown before the CSS is done loading, and one way of fixing the issue is using a `ready-to-show` event and waiting to show the `BrowserWindow` until after the CSS is done loading.

[More detailed instructions can be found here.](https://www.christianengvall.se/electron-white-screen-app-startup/)

## Related Issues

I filed a fresh bug report because no one has previously created a bug report about the white flash at startup. However, people have mentioned similar behavior ambiguously or in slightly different contexts, which I will link below:

* [Light theme is used before setting the dark theme](https://github.com/vector-im/element-web/issues/4895) (not quite the same issue because it involves the loading screen using the wrong theme)
* [White screen of welcome when upgrading Riot can be confusing](https://github.com/vector-im/element-web/issues/9243) (extremely vague bug report that may or may not be the same thing)
* [Element starts with white screen](https://github.com/vector-im/element-web/issues/14916) (exactly the same issue I'm seeing, but closed because it was blamed on Snap)
* [Loader looks bad in dark mode](https://github.com/vector-im/element-web/issues/16498) (also about the loading screen theme, not the blank screen before the loading screen)
* [Use system theme by default](https://github.com/vector-im/element-web/issues/18203#issuecomment-885629738) (white flash mentioned in comment thread)