improve clarity that the dashboard is loading data and not broken

[gruns]

right now in the UI it's too subtle/nonobvious that the page is loading data and the user should wait for that data to load. when loading many days worth of data, like 7+ days, the data can take a long time to load. for example, it just took so long (4-5 seconds) to load https://dashboard.saturn.tech/address/all?period=7+Days that i thought something was broken

potential fixes:

• in the middle of each panel, show a larger, more prominent spinning circle animation
  

• add a loading indicator to the overview panel, in the upper left, which is the first panel people look at, because its in the upper left, but lacks any indication that the data is loading

the text fields can also say Loading... instead of 0 [of thing]

note: the current UI will be change significantly by #38, but the same issue will persist there and thus needs to get solved regardless

of course one better alternative is to make all the data load lickity split quick. but that's likely a more substantial effort