Gopass recipient management

[jonas-pietzsch]

A new screen shall offer everything related to recipient management like documented from Gopass: https://github.com/gopasspw/gopass/blob/master/docs/features.md#managing-recipients

1. Show the list of recipients (gopass recipients)
2. Provide an action to update (recompute) the recipients' checksums (gopass recipients update)
3. Provide an action to add/authorize recipients (gopass add or gopass authorize)
4. Provide an action to remove/rm/deauthorize recipients (gopass remove|rm|deauthorize)