Fixes broken layout of web editor page on mobile

Fixes #13987 

<p><img src="https://gitlab.com/gitlab-org/gitlab-ce/uploads/3a58e7b576c25d5929bc1cdcbe163161/Screen_Shot_2016-03-10_at_1.13.19_PM.png" alt="Screen_Shot_2016-03-10_at_1.13.19_PM" width="487"></p>

See merge request !3167