Centralize loading component

[gjvera]

Currently we have loading components that all use material-ui's LinearProgress, however they are all implemented differently.
Some declare a component and then return LinearProgress

revelio/src/main/webapp/result-forms/route.js

Line 25 in 64373a2

const Loading = () => {

and some just return LinearProgress directly

revelio/src/main/webapp/user.js

Line 93 in 6f66f11

if (loading) return <LinearProgress />

We should centralize it so all components grab it from the same place and return the same loading component. This will make it easier if we ever wanted to change the component.

[willwill96]

We should make sure that we don't couple every loading state to a single loading component. I think we should have multiple centralized loading components for different use cases. i.e. routes shouldn't necessarily be using the same loading indicator as network retries