signal report loading states + optimistic loading

[adboio]

problem

when opening a signals report:

1. the data takes a long time to load (maybe this is just a dev thing? but i'm in ph code prod)
2. the loading states are very plain

suggestion

1. can we preload this data per report in the background once the user hits the inbox and cache it? maybe load for each signal that is visible in the list or something
2. let's add a nicer skeleton loader or similar here instead of just the static "Loading..." text