Fix broken redirects

If there's a full URL set in redirect_to, take it into account.

Test redirect_to with:

• Full URL ending in .md is used as is (.md is not converted to .html), for example if we want to use a full URL pointing to a Markdown file on GitLab.com. An edge case, but nice to have. I tested locally with redirect_to: https://docs.gitlab.com/charts/index.md.
• Full URL ending in .html - Tested locally with the charts redirects https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26159, for example redirect_to: https://docs.gitlab.com/charts/index.html works as expected.
• Full URL ending in / - http://fix-redirects.178.62.207.141.xip.io/ce/development/ux_guide/
• Relative URL ending in .md - http://fix-redirects.178.62.207.141.xip.io/ce/profile/README.html
• Relative URL ending in .md#anchor - http://fix-redirects.178.62.207.141.xip.io/ce/ci/autodeploy/index.html

Old method still works:

• gitlab-basics/create-group.md contains only the old method: This document was moved to [another location](../user/group/index.md#create-a-new-group). - http://fix-redirects.178.62.207.141.xip.io/ce/gitlab-basics/create-group.html

Misc:

• http://fix-redirects.178.62.207.141.xip.io/ce/development/documentation/ doesn't redirect. This was previously a bug that led us to revert https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/380#note_123324301

Fixes https://gitlab.com/gitlab-com/gitlab-docs/issues/304