Migrate to bootstrap


**This pull request fixes the following issues**:

- no ticket in general available

**You can preview the changes within** `http://192.168.2.83:8000/en/addons/accordion/`:

- [x] responsive behaviour tested
- [x] browser and device tested
- [x] passes validation (html / bootstrap)
- [x] SEO optimized (html outline)
- [x] printability (print css)
- [x] doesn’t break anything else

**Additional information**:

- none