Make the version dropdown links point to the document you're currently on

Part of https://gitlab.com/gitlab-com/gitlab-docs/issues/238 // gitlab-com&20 (closed)

Implementation

• Use Nanoc's <%= @item.identifier.without_ext + '.html' %> appended to the links in https://gitlab.com/gitlab-com/gitlab-docs/blob/bc09e8d46ec5d65193123b5fecb255cf45b978a6/layouts/header.html#L20-23.
• Mention in the 404 page the potential reason a page might not exist, see drawback.
• Make the "Go back" link a button.

Drawback

But then, this has some shortcomings: if a page is new, it won't exist in previous versions, so having the full URL would lead to 404. Also, what if the page existed in the old version but has since been renamed? This is difficult to solve with Nanoc's full URL solution, because we have to check if a file exists in a branch every time.

Example

1. I am on https://docs.gitlab.com/ee/user/project/pages/index.html
2. If I click on the 11.3 version I get to https://docs.gitlab.com/11.3/
3. I should go to https://docs.gitlab.com/11.3/ee/user/project/pages/index.html