Figure out how to configure auto-completion automatically on install

It would be really handy to have this. We had the following `postinst` script, but it doesn't work for unknown reasons:

```bash
#!/bin/bash
set -e
source <(life --bash-completion-script `which life`)
life --help
```

Start by reproducing the shown postinst script during installation and inspect how the PPA package invokes it. Investigate why sourcing `life --bash-completion-script` fails, then verify that a fresh install enables auto-completion without manual setup.

Written by the indexing model from the issue text.