Work around meteor less compiler limitation.

When you have multiple .less files in a project, they all end up getting compiled together fine in the resulting .css file, but you can't refer to things in one from another.  

For example, the only place that .hidden-xs is defined is within the (old) custom.bootstrap.less, but within my styles.less file I couldn't mix it in.

This way the base styles can be @imported too, and there's really just one less file.