web: set up journal for yesod devel, store it in App (fixes #101)
The web app's journal state is now kept in the yesod App as an IORef,
instead of using io-storage.
yesod devel now works; it uses the journal file specified by
$LEDGER_FILE, or ~/.hledger.journal.

web: update journal state handling, fix yesod devel - WIP