Handle unsupported code hosts

Created by: malomarrec

Problem

Some customers have unsupported codehosts (that is codehosts outside of GitHub, GitLab, Bitbucket). When compute-powered Batch Changes are turned on by default, we'll need to handle those gracefully

Proposal

• high priority: For customers that only have unsupported codehosts: the feature is off by default.
• medium priority (low priority if feature-flagged off by default): For customers that have a mix of unsupported and supported codehosts: the behaviour for results on unsupported codehosts is clear. Probably: they are omitted in the UI, with a message.