IDOC: Checkpoint for the frontend

Hey,
I have talked during the past few days with @bcipolli and we decided to merge my branch. It allows us to have a code review of the changes I've made. In the past, we used to merge changes on my branch and it was kind of a painful process. I think that merging with the develop branch and making small pull requests is the way to go. The development will go faster and allow other people to participate. Moreover, a lot of people want to make small HTML changes and having bootstrap will help them a LOT.

@jamalex Right now I've implemented Grunt.js for the compilation process. I think that implementing django-compressor should be another pull request.

What is the problem you're trying to solve?
- Have a better stack on the frontend
- More control on the css
- Cleaning up the central server files
- Allow people to create templates easily
- Make a better experience for new frontend devs

What is the design of your change?
- Integration of bootstrap in less in the assets folder
- Refactoring of the css to less
- Implementation of Grunt.js for the compilation
- In Debug mode, less.js will compile to css (the css files are cached in modern browsers). In the production mode, the template will use the css generated by Grunt.js
- I have used the [BEM](http://csswizardry.com/2013/01/mindbemding-getting-your-head-round-bem-syntax/) writing convention for the css. Read the article it's pretty straightforward.

How did you test the change?
- `grunt` to compile to css. If you want to test grunt.js you should install [node.js](http://nodejs.org/) and run `npm install` in the directory to install the packages.
- Run the webapp on the central server (local_settings.py)
- Edit style_with_bootstrap.less to make changes
- If you feel adventurous you can change the bootstrap.less file to add more stylings (forms, buttons...)

I know it's a lot of changes. Feel free to ask questions.
EDIT: npm install
If you have trouble with grunt: run `npm install grunt --save-dev`