Bootstrap as submodule

Do you think it's acceptable to add the original [bootstrap repo](https://github.com/twbs/bootstrap) as a submodule? It allows us to keep our modifications in a single variables.less file, which makes adjusting things easier. We don't need do always use the online customizer.
Disadvantage: It's a lot of files that won't be used (only the less subfolder is relevant).

Please do NOT merge yet. There's still some changes to come, I just want some early feedback.