Issue title: Export modal is open by default on mobile devices
 Issue body: On my iPhone using Safari, the export modal is open, and empty, when the page loads. Additionally the dialog cannot be closed. This makes the site unusable on the iPhone and maybe other mobile devices.

@ziyafenn Take a look at this and let me know if you can fix. I may have to roll back #5 if not. My apologies for not checking before merging this.

![IMG_521751D7D90F-1](https://user-images.githubusercontent.com/8269046/90387822-7a2b0f00-e04c-11ea-83fb-350a1fbd086b.jpeg)

 Comments: 
 Comment 0: @edelstone i'll take a look tonight. I'm working now on a sketch plugin that can use the exported JSON to create a color palette inside Sketch. I'll create an issue for that too :)
 Comment 1: If the tabindex stuff you did on that last commit in #5 is causing the problem, just revert it and I'll make a new issue to investigate tabbing accessibility across the entire app. That's probably a better strategy. 
{'source': 'GithubIssues', 'repo_name_with_owner': 'edelstone/material-palette-generator', 'issue_id': 680135307, 'repo_stars': 111.0}