Migrate linting from standard to XO

[jfmengels]

Migrate linting from standard to XO, to have a consistent linter and style accross the Zeit projects, as requested by @leo.

Notes:

• I also added 'xo's react config for this project.
• There are two proptypes that seem to not be used explicitly. From what I gather, they are accessed and used dynamically, so I ignored the warnings there.

[leo]

Thanks! But why do we need to use babel-eslint as a parser? Isn't esnext: true enough?

[jfmengels]

xo supports jsx syntax out of the box, but you need babel-eslint to support not-yet standard features. In this case, you're using static class properties, which is not yet supported by the default parser.

[leo]

Ahhhh. Thanks! 😊