Add a UI library

We started this project with @brianflanagan's design, and its minimal and clean style has been approved by many.

While the design itself is excellent, I would like to refactor how it's implemented in the code.

Right now: A collection of SASS files under fether-react/assets/scss/, all compiled into one huge CSS file, imported by react.

Problem: It's very hard for non-UI developers (read: the whole Parity UI team) to add new components to it. E.g. I wanted to add a on/off toggle button once, I had no idea how to do that.

Solution: Add a UI components library to this project, like Semantic UI React (also used in shell, polkadot...). We would override their default theme to match the current Fether theme (maybe with css-in-js?). After that, adding new components like toggle buttons would be easy.