Use skeleton screens instead of loading spinners where applicable

[johanstokking]

Summary: Spinners often result in suboptimal UX, because they leave the user in the dark about what to anticipate from a loading event (among other things, see e.g. here for more info) It's often times better to provide a skeleton of the component/content/view that is being loaded, possibly in conjunction with a spinner or similar animation that indicates loading.

What is already there? What do you see now?

• A loading spinner

What is missing? What do you want to see?

• Skeleton graphic/markup for components that load frequently
• A flag on these components that will result in the skeleton being rendered (for loading events)

cc @bafonins

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Original issue: https://github.com/TheThingsIndustries/lorawan-stack/issues/1223 by @kschiffer

[kschiffer]

I don't think this will ever have enough importance to be tackled and it's really a cosmetic thing only. I'll close it and maybe reopen if I should deem this more relevant in the future.