Show "Provide feedback" link on the homepage

Problem

We're introducing a new homepage for all GitLab users. We want to ensure that people can provide feedback on the experience, so we can use that feedback to continue to improve the offering.

Proposal

Follow the pattern on the to-do list, and introduce a "Leave feedback" link on the new homepage when it's introduced. The feedback issue we will link to --> #553938.

Note: this feedback issue is currently confidential. This is okay for our internal release but, we'll need to update the link to a public feedback issue when the homepage is rolled out externally.