e2e tests for all code hosts

Created by: felixfbecker

• github.com
• Github Enterprise
• Phabricator (native)
• Phabricator (extension)
• Phabricator w/ staging repos
• Bitbucket (native)
• Bitbucket (extension) 🏃 @felixfbecker
• Gitlab

Blocked by: https://github.com/sourcegraph/sourcegraph/issues/5286