Rework `user` subcommand

No args: do the same as `health` now

Subcommands:
- clear: prompt to confirm, then delete user's db file. Replacing `--clear`
- view: show the user's star table
- add: do what I just implemented `auth` for, but prompt for a label too
- list: show the registered users
- switch: pull up a list and let the user select one 
- remove: switch but delete user instead