added focus on close button to license modal

[bblarney]

Overview

Closes #6137

Description

• The close button is now in focus on the 'License and Open Source Notes' modal within the 'About Github Desktop' modal. Old behavior would open new window on enter key press inside the modal, new behavior closes the modal. I used the code and comments from the T&C file for consistency.

New behavior:

1. Open "About GitHub Desktop"
2. Click on "License and Open Source Notices"
3. Click Enter/Return
4. "Close" button invoked and the modal closes.

Any changes needed let me know! Thanks

Release notes

Notes: no notes

[shiftkey]

@bblarney thanks for opening this up!

I haven't had a chance to review it yet, but I wanted to point out that the email address used in the commits isn't found in your GitHub account (so you won't be credited as a contributor on this page if we end up merging the PR).

[iAmWillShepherd]

Thanks for your contribution!