Add the `li backup` commands with `export`, `import` and `verify`:
- data types selection (`-i`/`-e`) and profile selection (`-p`), `--no-db` on export, `--use-db` on
import
- GPG symmetric encryption of the archive (`--encrypt`)
- progression is shown, with ETA
- the commands can run without a backend connection (import from DB MUST be run without
backend)
- cli (base): `BRIDGELESS_COMMANDS` holds the name of the few commands which can be run
without backend.
rel 480
