feat: Relates to #158. Add styled-components UI library. See commit details.

* Add styled-components dependency to start introducing it to components instead of Sass

* Wrap App.js with styled-components ThemeProvider and inject `fetherTheme` prop colours so they are available to child components.

* Added styled-components' GlobalStyle https://www.styled-components.com/docs/faqs boilerplate as child of ThemeProvider so global styles may be defined here

* Replace assets/sass/layouts/_wrapper.scss with styled-components including:
  * DivContent is declared in App.js since it is only used here
  * DivWindow is declared in App.js since it is only used here
  * DivWindowContent is declared in shared folder since used by different components

* Removed assets/sass/layouts/_wrapper.scss. Noting that `.connector` class is not used

* Use styled-components in TxForm and TxDetails components (that were introduced in PR #307).

* Add a style.js file in the same directory of the component where styled-components are used

* Add styled-components theme animations folder with a SlideInLeft animation and introduce a theme to TxDetails

* Add keyframes animation to TxDetails component (i.e. slide-in animation that's used in the details section that appears when you click Details/Hide button on the Send Ether/THIBCoin page)

* Fix `faint` style implementation to use `rgba` with styled-components

* Note that I decided not to use defaultProps feature of styled-components as it bloats the code too much

* Note that currently we are using Sass _variables.scss for colour variables, and since many components still use some of the Sass variables we cannot remove them yet.

* Note that the original branch where this work was done is https://github.com/paritytech/fether/compare/luke-293-show-tx-fee-styled-components?expand=1, but since multiple PRs have since been merged it was easier to just code again using it as reference

* Disadvantage of styled-components appears to be when you go to debug components in Dev Tools > Elements. The classNames appear as a hash `<div class="sc-EHOje dGUdfn">` instead of the classname!!