Add setting to enable read-only mode for automation for all Sourcegraph users

Created by: christinaforney

https://app.hubspot.com/contacts/2762526/company/608245740 wants to send out manual campaigns to their entire organization. This means that all Sourcegraph users on their instance must be able to view campaigns in a read-only mode. We have confirmed that they do not have any repository restrictions, so it is ok for there to be solely two different roles:

1. Admins (current functionality) - can perform all operations with Campaigns
2. Users - can view campaigns in read-only mode. This is a setting that can be enabled and is off by default (not all customers will want to grant everyone access to everything in this way).

We will obviously enhance this functionality in the future, this is just a simple case to start and solve an immediate need for a customer.

See slack thread for additional context.