Add 'Play a friend' button

[programarivm]

When clicking the button, a popup should be displayed showing the user a link to be shared with a friend.

For further information please visit:

• Implement 'Play a friend' mode chess-server#1

• Refactor the server's response to a /playfen request chess-server#3

[CHuante]

hi, id like to call dibs and take a crack at this and contribute if its still around?

[programarivm]

thank you @CHuante, I can't wait to see what you come up with!

[CHuante]

So for clarification, what should the button do? and where do you want it?

[programarivm]

The button should be somehow similar to "Play with a friend" button at https://lichess.org.

It should be added to components/Button, which is a new component created a few hours ago.

For the sake of simplicity though we're creating a randomish token appended to an URL to be shared with a friend to play.

[programarivm]

@CHuante probably this issue is not too obvious since it implies some knowledge on the chess server actually. Sorry if it isn't a good first issue.

If this is okay with you, please feel free to write a comment on #10 so I can assign to you that one instead.

See isaacs/github#100

Thank you!

[programarivm]

Merged PR #17