Jenkinsfile: Publish release artifacts for release branches.

Previously, the Jenkinsfile did not publish anything except for the
repositories: the release tarball would not get copied to
https://dl.jami.net for stable channel, and no commit nor tag would be
published for the release.

This change addresses these shortcomings with the following changes:

1. Tarballs can be published for the stable channel.
2. Release commits/tags can be published to their corresponding
branch (stable/nightly).

The RELEASE_VERSION string is also once again derived in a
deterministic fashion, from the date of the last commit.

* Jenkinsfile (SSH_PRIVATE_KEY): Remove variable.
(RING_PUBLIC_KEY_FINGERPRINT): Rename to...
(JAMI_PUBLIC_KEY_FINGERPRINT): ... this.
(GIT_USER_EMAIL, GIT_USER_NAME, GIT_PUSH_URL)
(JENKINS_SSH_KEY, DL_SSH_KEY): New variables.
(params.DEPLOY): Fix description.
(Checkout channel branch): New stage.
(Generate release tarball): Also commit and tag.
(Publish release artifacts): New stage to publish conditionally based on the
DEPLOY parameter and the selected channel.
(Sign & deploy packages): Use the 'sshagent' step to setup SSH access.
<--remote-ssh-identity-file>: Remove argument.
<--keyid>: Adjust variable name.
* scripts/deploy-packages.sh (package_snap): Simplify.

Change-Id: I9008ecc2a4ef9820dbc96e26c966ae72110d897d