blob view for rich content is too narrow on mobile

Summary

blob view for rich content (e.g. .md markdown and other server-side rendered contents) is too narrow on mobile.

Steps to reproduce

Access to https://gitlab.com/gitlab-org/gitlab-ce or more clearly https://gitlab.com/gitlab-org/gitlab-ce/blob/master/README.md

What is the current bug behavior?

As side margins of blob viewer for rich content are too large (20%), blob view comes too narrow especially on mobile (smaller devices).

What is the expected correct behavior?

For smaller devices, no side margin is expected to maximize the readability of rich blob.

Relevant logs and/or screenshots

Output of checks

This happens on GitLab.com.

Possible fixes

Use media query for the relevant CSS.

cf.

• introduced in d0bb0d6e in !12198 (merged)

/label bug UX rich blob viewer