Update GitHub App instructions for org-owned repos

[hectorsector]

Since our courses are packaged up and reused by Learning Lab customers, it is likely this course will be reused privately (so learner repos will be owned by the organization). When this happens, the first few instructions will need to change considerably. At first glance, these changes would be:

• installation of the TravisCI GitHub App may not happen by the individual, but will be done by the organization admin
• need some way to communicate additional setup is required to the customer when they try to create this course on their org (could be as easy as including release notes?)
• According to the docs, as long as a GitHub App doesn't require org-level access, repo admins can install it in their own repo. We need to verify if the TravisCI's app permission indeed allow an individual to install it themselves so we can avoid the first two bullet points above
• consider whether using commit statuses in lieu of the GitHub App is a possibility (I think Travis is planning to deprecate this?)