Add user agent and ignore archives

Changes to cmbr:

• Ignore URLs that have archive formats (\.zip|\.tar.gz|\.tar.bz2|\.tar) in them per gitlab-org/gitlab#338978 (comment 748768951)
• Add GITLAB_USER_AGENT to bypass CloudFlare check on Staging that determines if it's a robot. We use an agent for GitLab QA pipelines and will set this env variable in cmbr as well. Validated locally that it fixes 503 errors like this https://staging.gitlab.com/gitlab-com/gl-infra/cmbr/-/jobs/39928821#L55
• Fixed Visited counter - it should be added to OnResponse instead of OnRequest. As OnRequest is done before the request is actually sent to the server. All credit goes to StackOverflow experts https://stackoverflow.com/a/51451414 😄