Running `securedrop-admin restore` without an argument should list available backup files

[nathandyer]

This is just a small quality-of-life feature that might be nice.

Rather than doing:

ls ~/.config/securedrop-admin
<copying and pasting a filename>
securedrop-admin restore <filename>

to restore a backup from an Admin Workstation, it would be nice if:

1. securedrop-admin restore listed the available backup files detected in ~/.config/securedrop-admin
2. Each item in the list was numbered, and
3. It prompted you for the number of the backup you wish to restore rather than making you type the full filename

[zenmonkeykstop]

TBH I kindof don't like they they get written to .config/securedrop-admin - IMO having it just be a path you specify (either absolute or relative to current working directory) in backup and restore actions would make more sense. Folks can rename the files too so listing them reliably might be problematic as they're basically just tarballs.