Add Pendo to GitLab.com (SaaS)

blocked by https://gitlab.com/gitlab-org/telemetry/issues/97

Overview

We need to add the Pendo snippet to GitLab.com (SaaS) so we're able to track and better understand user behaviour throughout the application.

Proposal

• Add the Pendo snippet into application.html.haml so it renders on every page.

  • See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14040/ for an example on how we might add this.

• Combine all Tracking under one checkbox. Remove the usage ping checkbox as it should be covered by the Tracking checkbox. See https://about.gitlab.com/handbook/product/#convention-over-configuration.

• Rename the Usage Statistics section to Telemetry - Usage Tracking

• Remove the Preview Payload option since that only covers usage ping and not any other tracking we might be doing. We should replace this with a link to our Telemetry documentation.