Put Less into math mode 'always'

The upgrade to v4 of Less in b154c7b
introduced some changes that broke our version of Bootstrap.
Specifically, division in `.less` files now has to be in parantheses by default.
We fixed a few instances of that in the vendored files,
but missed many others.

Instead of futzing around with these files more,
this patch referts these changes, and puts Less in a compatibility mode.
**Note**: This mode is deprecated, so this is only a temporary solution,
until we find the time and money to update Bootstrap.

frontend/Gruntfile.js

@@ -17,6 +17,7 @@ module.exports = function (grunt) {
                 syncImport: true,
                 compress: true,
                 sourceMap: true,
+                math: 'always',
                 sourceMapURL: 'style/style.css.map',
                 sourceMapBasepath: 'style'
             },