Add a simple pair of tools for blocking and unblocking deployments

The main use-case here is for ensuring that we don't deploy to master
while doing demos.

(imported from commit d3c8ba502052b352291548200032f39e0743b774)

tools/block-deployments

@@ -0,0 +1,3 @@
+#!/bin/bash
+
+exec ssh zulip@staging.zulip.net 'mkdir deployments/lock'

tools/unblock-deployments

@@ -0,0 +1,3 @@
+#!/bin/bash
+
+exec ssh zulip@staging.zulip.net 'rmdir deployments/lock'