Replace the white screen while the PWA is being injected

[Poliuk]

Add an animation so the user doesn't see a blank page. Some times the injectors takes some seconds to load.

The animation should be injected between lines 61 and 62.

https://github.com/worona/worona-wp-plugin/blob/28f52d1515d2aec61f118414f28ec1077b2f78b8/injector/injector.js#L61

https://github.com/worona/worona-wp-plugin/blob/28f52d1515d2aec61f118414f28ec1077b2f78b8/injector/injector.js#L62

[Poliuk]

@luisherranz suggests to add a loading animation with a 500ms delay. This way, the animation is only visible in the cases where the PWA is taking to much to load.