Add dough styles

This is related to the Dough PR **must be merged first**: https://github.com/moneyadviceservice/dough/pull/208

- Adds Dough component styling that is specific to MAS
- Adds reference to [Yeast](https://github.com/moneyadviceservice/yeast), to add shared mixins (tools use these as well as frontend) that were in Dough but are now not