WIP: Slack application landing page

Open Valery Sizov requested to merge slack_landing_page into master

What does this MR do?

This is a landing page for GitLab Slack application

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/34720

Edited by Valery Sizov