Fix selection and mint pages
The core of the issue was that the "Mint DOI" button performed a full navigation, rather than an SPA navigation, which meant that the Redux store was cleared and the selected datasets lost. In the past, this issue was "hidden" by the fact that the store was being persisted.
I've fixed a number of other issues and warnings, and I've spotted a few more that I'll work on this afternoon. I've also refactored the SelectionPage and MintSelectionPage with modern React.