[CLOUD-202] Enforce feature flag check for the GitHub App setup handler
Created by: unknwon
This PR adds a feature flag check for the Sourcegraph Cloud GitHub App setup handler.
An error is displayed for organizations without having the feature flag:
(the response string in the screenshot is outdated, but the behavior is the same)
Testing
For testing locally, please follow the local testing guide.
Jira: CLOUD-202