Replace fullscreen loading indicators with skeleton UI

Throughout the web app, when you navigate to a different view or perform an action, a fullscreen loading indicator appears that blurs the background and shows a loading spinner. This is distracting, disorienting and makes the app feel slow. When something is loading, only the elements that changed should have a loading state (for example the view underneath the header and tab bar, or a card for a new check being added to a benefit), and ideally this should be a [skeleton UI](https://yeon-design.tistory.com/52).