Admin page has its wider content area on mobile

Summary

Every page under Admin (/admin) has its wider width in the main content area when you're on mobile (typically less than ~ 500px viewport).

Steps to reproduce

1. Log in to gitlab with admin on mobile (5-inch smartphone, not tablet) and have access to https://gitlab.exaple.com/admin where gitlab.exaple.com is your gitlab instance domain.
2. Scroll right in the main context area to see extra blank on right.

What is the current bug behavior?

The main context area is scrollable to right and has extra blank on right.

What is the expected correct behavior?

The main context area should not be scrollable to right.

Relevant logs and/or screenshots

Output of checks

master as of 2017-06-29 (e.g., 8c7a88d6)

Links

cf. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12325#note_33664347

/label bug UX