Show a 'loading' state indicator for the asset overview UI

[omnat]

What problem are you trying to solve?
Tokens from my account seemed to have disappeared while I was switching ETH and token views. This is a problem because for those 1-2 minutes I thought I lost them while they were loading.

Describe the solution you'd like
A loading state when the data is getting updating on UI. Solution is same as suggested by Christian in issue #3776, just different context.

[dpazdan]

Another solution to this could be a "last updated on" date/time. The extension would always keep cache of the last info it has, for immediate display, then once it updates, it updates the "last updated" date/time with it.

[bdresser]

Also from support sync today: sometimes ETH balances from Infura are slow to load, so this could be helpful for users' main balance too.