Allow the sidebar to be made wider on smaller screens

Report https://about.gitlab.com/2016/01/15/making-gitlab-better-for-large-open-source-projects/#comment-2477979768

When you collapse the sidebar manually on a large screen you get a > to expand it.

Currently on a small screen it is collapsed automatically and you can't expand it.

You should be able to collapse and expand on a small screen too.

/cc @jschatz1