Feature/2554 build the perseus less files

Hi @aronasorman - this is for #2554 

Branch **Develop**

I have compiled the `.less` files already but some .css files at the `perseus/stylesheets/exercise-content-package/widgets/` folder didn't compile.  Here are two samples:

```bash
(ka-lite)cyril-lappy6:kalite cyril$ lessc distributed/static/perseus/stylesheets/exercise-content-package/widgets/categorization.less
NameError: .draggy-boxy-thing-hover is undefined in /Users/cyril/w/fle/ka-lite/ka-lite-src/kalite/distributed/static/perseus/stylesheets/exercise-content-package/widgets/categorization.less on line 36, column 13:
35         &.target .cards-list.cards-area {
36             .draggy-boxy-thing-hover(#999);
37         }
...
(ka-lite)cyril-lappy6:kalite cyril$ lessc distributed/static/perseus/stylesheets/exercise-content-package/widgets/expression.less
NameError: variable @baseFontFamily is undefined in /Users/cyril/w/fle/ka-lite/ka-lite-src/kalite/distributed/static/perseus/stylesheets/exercise-content-package/widgets/expression.less on line 130, column 16:
129 .perseus-widget-expression-old > .output > .placeholder > .error > .message {
130     font: 12px @baseFontFamily;
131     line-height: 1.4em;

(ka-lite)cyril-lappy6:kalite cyril$
```

I saw that they're declared under the `exercise-content-package/variables.less` file but I have no idea how to cross-compile them.  Do you happen to know how?

Will read more on these.

/cc: @rtibbles