Add to troubleshooting how to get help about GitLab

When a user creates an issue about a problem not related to omnibus-gitlab, we need to explain how a user can get help.

Add under troubleshooting a section that says something like:

"If the installation went without problem and configuration files got updated and you still get a 500 error then it is most likely a problem with GitLab itself rather than package problem. See "link to getting help" for ways to get help. Please mention me if you think this is a problem caused by package."

Also create a issue response template to speed up responding in the issue tracker.