Server side batch changes beta

Created by: jyavorska

Criteria for moving to beta:

• we get feedback that server-side batch changes is better than the CLI for creating large scale batch changes
• it scales up to the scale of a customer like https://github.com/sourcegraph/accounts/issues/565 (~10k repositories)
• we think that the maintenance and installation burden is manageable

This is not a complete criteria, but we will also want to make sure to have a detailed list of what we support (in terms of infrastructure) and limitations before moving to beta.