🍰 Add migration plan and frontend code guidelines to our docs

Created by: Tirokk

Authored by alina-beck Oct 29, 2019 Merged Nov 4, 2019

---

🍰 Pullrequest

• adds a migration plan to the webapp/README.md
• adds code guidelines for components, HTML, SCSS, Vue
• adds more info about frontend testing

Issues

• related to #2074

Todo (not necessarily in this PR)

• add Vuex guidelines
• add GraphQL guidelines