Add setting for number of concurrent builds.

Includes a little rework of the settings page to fit in an input
field.
Merge branch 'master' into mm-concurrent-builds-setting
Mark invalid concurrent setting input properly.

This adds a CSS class if the content is invalid.
More subtle coloring for invalid fields.
Show both labels on travis-switch
complete fix of all switch updated styles
Merge branch 'switches-styling' into mm-concurrent-builds-setting
Concurrent builds have got some STYLE
Fix specs
Merge pull request #289 from travis-ci/mm-concurrent-builds-setting

Add setting for concurrent number of builds