Disable Manifest All button and show loading indicator when manifest is requested

When manifesting at the end of day, clicking the 'Manifest All' button does not make it clear enough that it was clicked and is processing. As a result, it is easy for a staff member to click it again, which results in the manifest being submitted to Royal Mail twice and subsequently being charged for postage twice.

Once the manifest is requested, the button should be disabled to prevent double-manifesting, and a loading indicator should be displayed to make it clear that the request is being processed.