Navbar and sidebar interfere with print styles

Summary

When printing issue and merge request pages, the top navigation bar and the sidebar (if displayed) interfere with the print style.

Steps to Reproduce

• Open the following popular issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/4232 or MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1921
• Display the right-hand sidebar, participants, milestone etc
• Check the Print Preview in Safari or Chrome (Ctrl +p or Cmd + p on Mac)

Expected Results

Nav and sidebars would not be displayed in print version

Actual Results

Nav and sidebars are displayed in print version.

• The sidebar will squash all other content to the left hand side of the print preview
• The sidebar appears on every print page (in Chrome)
• The navbar will appear on each page overlapping other content (in Chrome)

Example from the issue page: