Add Origin/Label/Suite to Debian mirror InRelease file

Keybase `InRelease` file on https://prerelease.keybase.io/deb/dists/stable/InRelease contains only `Codename`:

```
Hash: SHA256

Codename: stable
Date: Tue, 25 Feb 2020 21:24:21 UTC
Architectures: i386 amd64
Components: main
```

Compare with for example Google Chrome mirror, that contains `Origin`/`Label`/`Suite`/`Codename`:

```
$ curl -s https://dl.google.com/linux/chrome/deb/dists/stable/Release |head -5
Origin: Google LLC
Label: Google
Suite: stable
Codename: stable
Version: 1.0
```

The latter allows users to easily configure `unattended-upgrades` to upgrade Google Chrome in `/etc/apt/apt.conf.d/50unattended-upgrades`:

```
Unattended-Upgrade::Origins-Pattern {
"origin=Google LLC,codename=stable";
}
```

Keybase should add the `Origin`/`Label`/`Suite` fields to allow users to tell repositories apart.

For more information, see: https://wiki.debian.org/DebianRepository/Format#A.22Release.22_files

Start by inspecting the Debian mirror's InRelease file at https://prerelease.keybase.io/deb/dists/stable/InRelease and compare it with the Google Chrome Release example in the issue. Read the Debian Repository Format reference, then verify that the generated metadata includes Origin, Label, and Suite so it can be matched by unattended-upgrades.

Written by the indexing model from the issue text.