Fix choose layout control

[mingiryu]

Description of proposed changes

What is the goal of this pull request? What does this pull request change?

• Allow user to change layout after the pause, but prevent the user from changing layout during the animation.

• Added handleChangeLayoutClicked.
  Defined a helper function to replace the anonymous onClicks functions that are currently in place. This will make sure that the loopRunning constant is valid when the onClick event happens.

Related issue(s)

Fixes #1047
Related to #1047

Testing

What steps should be taken to test the changes you've proposed?

1. Open localhost:4000/zika
2. Start the animation.
3. Try to change the tree layout (e.g. to radial). You shouldn't be able to.
4. Pause the animation before it finished.
5. Try to change the tree layout (e.g. to radial). You should be able to.

If you added or changed behavior in the codebase, did you update the tests, or do you need help with this?
I don't think there's a test for this behavior.

Thank you for contributing to Nextstrain!

[jameshadfield]

Tested and works well. A helper function is definitely cleaner. Thanks!