Get rid of the white flash when the app starts up (#3083)

* Prevent FOUC on startup, use Signal Blue instead

Instead of flashing a white background on startup, the app will show a blue background while everything is loading.

* Make sure tests are rendered on white background

* Use `icon_250.png` as the loading image

* Remove hardcoded 'Loading...' text from template

main.js

@@ -210,6 +210,10 @@ function createWindow() {
       minWidth: MIN_WIDTH,
       minHeight: MIN_HEIGHT,
       autoHideMenuBar: false,
+      backgroundColor:
+        config.environment === 'test' || config.environment === 'test-lib'
+          ? '#ffffff' // Tests should always be rendered on a white background
+          : '#2090EA',
       webPreferences: {
         nodeIntegration: false,
         nodeIntegrationInWorker: false,