backups (tgz): save integrity information

we generate a signing key pair for each target. Initially, I had this
as global. We needed a route to return the public key and putting it
under backup target seemed natural. Since we delete the backups when
we delete a target, we lose all the signing hashes. So, it's fine to lose
the key pair on target delete.

src/platform.js

@@ -177,7 +177,7 @@ async function initialize() {
     // we remove the config as a simple security measure to not expose IP <-> domain
     const activated = await users.isActivated();
     if (!activated) {
-        debug('start: not activated. generating IP based redirection config');
+        debug('initialize: not activated. generating IP based redirection config');
         await safe(reverseProxy.writeDefaultConfig({ activated: false }), { debug }); // ok to fail if no disk space
     }