Slack application landing page

MR merge date extension to 14/11/2017

CE backport: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15354

I would like to extend the merge date for this deliverable 1 week.

Sign off:

• @stanhu (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2415#note_46532082)
• @smcgivern
• @DouweM

Tasks left:

• Green pipeline - @lbennett (waiting on !3315 (merged))
• BE reviewer review - @felipe_artur
• BE maintainer review - @smcgivern
• FE maintainer review - @jschatz1

Let's do this! 💪 🏆

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?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/3027