Replace rails-assets-bootstrap with bootstrap-sass

In order to use bootstrap variables, mixins and all the power of sass + bootstrap, we need to use sass version  instead of precompiled one.

- [x] Remove rails-assets-bootstrap
- [x] Add official [bootstrap sass gem](https://github.com/twbs/bootstrap-sass)
btw, could you please also remove normalize-css as it's already bundled in bootstrap?