Add new project custom help text option

Similar to how one can define custom help text to show on the help page lets allow admins to define some extra text to display on the new project page.

One use case is information about the use of access levels and add links to project policies etc.

Proposal

Add a new markdown text area to the “Appearance” admin page to define this text at the instance level. The text appears in the left column of the “New project” page, after the default text.

Example: see images in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15541