Add GITLAB_ENVIRONMENT_URL

Problem

Currently, when an application is using ~"auto devops" there is no way to know the URL of the environment you're running in. This information is required when a Rails application is using ActionMailer so that config.action_mailer.default_url_options.host can be set.

Proposed Solution

Create a new environment variable named GITLAB_ENVIRONMENT_URL. This variable would be populated from the Helm chart value gitlab.envURL which is populated from the CI variable $CI_ENVIRONMENT_URL.