Improve layout and UI loading states

Sorry for the diff...

• Re-add site title and make nav and breadcrumbs bars fluid so they take up the whole screen.

• Clean up and improve footer (don't use a nav element, use custom CSS, centre logo+text)

• Refactor and improve Loading component (rename to Loader, don't use absolute positioning, use CSS spinner, add variants for flexibility: inPanel, spacedOut, etc.)

• Replace all occurrences of react-loader-advanced, which used to add an ugly overlay to elements while loading, with the refactored Loader component.
• Fix a few bugs here and there (e.g. the form to add beamline managers used to allow submit with no user/beamline selected).