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)
it's here:

* https://github.com/twbs/bootstrap-sass/blob/c9f2e89f826db06e058ac2a97da4dd7fd6b10ccc/assets/stylesheets/_bootstrap.scss#L6
* https://github.com/twbs/bootstrap-sass/blob/c9f2e89f826db06e058ac2a97da4dd7fd6b10ccc/assets/stylesheets/bootstrap/_normalize.scss