add chi router for handling delete requests

adds the delete handle for delete a specified ssh key.

appview/state/router.go

···
 		r.Use(AuthMiddleware(s))
 		r.Get("/", s.Settings)
 		r.Put("/keys", s.SettingsKeys)
+		r.Delete("/keys", s.SettingsKeys)
 		r.Put("/emails", s.SettingsEmails)
 		r.Delete("/emails", s.SettingsEmails)
 		r.Get("/emails/verify", s.SettingsEmailsVerify)