Deploy executors for demo.sourcegraph.com

Created by: malomarrec

We want to dogfood using server-side batch changes, and we also want to start demo-ing it to customers. We cannot use sgdev for demos, because it contains private code (and CEs don't use it).

Proposal

Deploy executors for demo.sourcegraph.com, that we can use in demos, with autoscaling.

This will be a good dry run for executors, and the setup docs!

Success criteria

• Malo (and others!) are able to run a few batch changes server-side every day on demo.sourcegraph.com.
• We reach a point where customer engineers are comfortable demo-ing server-side instead of locally