Make the cleaner available to users by exposing it as a subcommand
to thelounge storage.
This is recommended to be run whenever the storage policy significantly
changes in a way that makes many messages eligible for deletion.
The cleaner would cope, but it'll be inefficient and can take many hours.
Due to how storage works in sqlite, the space would not actually be
given back to the OS, just marked for future writes.
Hence this also runs a vacuum to compact the DB as much as it can.
This introduces the ability to run the migration offline, while
TL is not running as the migrations can take a long time.
The migrate command is added as a `thelounge storage` subcommand.
Reason being that it is expected that more subcommands will follow,
say `thelounge storage clean` to remove partial data from the db.
