Add preliminary support for sessions (WIP)
Add more Session methods

- Move session totals into `total` subtable
- Stop current session on logout
- Move each sub-command of the main slash command into its own function
called by the main handler
- Move ID slash command help into its own function
- Change spam subcommand to handle current session and total spam
separately
- Add session subcommands
- Add date subcommand (NYI)
- Add metatable to `L` (localisations table) to detect unlocalised
phrases
- Finish implementing `UpgradeDB` function