New: session packs.

A client can now have more than one session balance, for example: separate packs for Reformer and Mat sessions.

Add a pack from the client's Sessions tab, with a name and a number of sessions.
Optionally link a pack to a session type. Sessions of that type can be logged manually or automatically from your calendar events.
Edit a pack's name, session type or number of sessions, or remove it. When removing a pack, you can move its remaining sessions to another pack or forfeit them.

Fixes:
Sessions logged automatically from your calendar now appear in the session history right away.
Deleting a session now adds back exactly one session.