Add loading gif on initialization and saving

[joeyklee]

• Loading those big models in the beginning can take some time. We should include a loading gif or something to indicate that the app is loading or doing some processing task

[owenmcateer]

I have created a loading screen to be shown as early as possible until all models are loaded and the App is ready.
Here is the simple "block" loading icon. https://github.com/owenmcateer/BodyBlock/blob/loadingScreen/src/common/loading.svg

PR: #23

[joeyklee]

#23 address the initialization, but we will still need to handle the loading icon on saving. We can address this in a follow up PR :)

Thanks @owenmcateer !

[owenmcateer]

Agreed, I've created a new issue to add state to the App #24