Allow non-admins to use SSBC

Created by: mrnugget

This fixes the first part of #27543 (closed) by allowing users to create batch specs for execution and to also view their own batch specs.

This implements the Stage 1 described here: https://github.com/sourcegraph/sourcegraph/issues/27543#issuecomment-987912774

I want to tackle the frontend work in another PR and the cache-access too.