NewSessionView: fix regression with module changes

0af425c has a regression where module
changes within the selected subject don't update the shown decks and
module stats. Fix it by re-refetching decks reactively on each change of
`moduleId` and use a key expression [1] to make sure deck elements get
reinstantiated and reinitialised accordingly.

[1] https://svelte.dev/docs/logic-blocks#each

Author: schu