Display nav controls on mobile

Set inputs inside .nav-controls to full-width on viewports below 600px

Fixes #13885

**Affected pages**

/dashboard/groups

/admin/builds

/dashboard/projects

/dashboard/issues

/dashboard/merge_requests

/dashboard/milestones

/dashboard/todos

/explore

/groups/{group-name}/issues

/groups/{group-name}

/groups/{group-name}/milestones

/{group-name}/{project-name}/builds

/{group-name}/{project-name}/forks

/{group-name}/{project-name}/issues

/{group-name}/{project-name}/labels

/{group-name}/{project-name}/merge_requests

/{group-name}/{project-name}/milestones

/{group-name}/{project-name}/wikis/home

/{group-name}/{project-name}/wikis/pages

Some Screenshots

**Groups**

![Screen_Shot_2016-03-15_at_2.22.23_PM](/uploads/b55348676693cc3ff67cddc4528f7f6c/Screen_Shot_2016-03-15_at_2.22.23_PM.png)

**Admin > Builds**

![admin-builds](/uploads/2e84119660f4895a1530883412c4b728/admin-builds.png)

**TODOs**

![todos](/uploads/84344c43011e22f902701038932d3eaa/todos.png)

**Dashboard > Projects**

![dashboards-projects](/uploads/f508d9107598038798aa10485d3d5cfe/dashboards-projects.png)

**Project Issues**

![issues](/uploads/d6c69a8f81ee07da95b2a3b99b81cf51/issues.png)

**Project Merge Requests**

![merge_requests](/uploads/b3c1a537f28931bd38077c36627a0d7a/merge_requests.png)

See merge request !3214