Replace the convoluted useAccountSession composable with a pinia store
that in addition allows for the consolidation of all session related
functions to grouped into one module.
To make it possible to render the timetable in the user's local time we
need to know the timezone to render it in on the server. Otherwise
there will be hydration errors and paint flashing as the client renders
a different timezone.
Add a server global default timezone that can be overriden on a
per-account bases to prepare for timezone handling the timetable.
