Hide Modal on unmount

[AndyHubert]

Please read and mark the following check list before creating a pull request:

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

This prevents the Modal from getting stuck open when unmounted in a visible state. Accords with Issue #718.

Closes #808
Closes #813

[artyorsh]

Thanks for opening PR.
Btw, did you able to test this? In my opinion, this can bring some inconsistency, for example, when visibility is handled by parent component, which means visible is passed to modal from the parent component state or maybe mobx / redux store

[AndyHubert]

I did not run tests. I was tripped up in the PR submission process you guys outline and did not have the time to trouble-shoot. However, I did run the new code on my project, where it worked as expected.

I am not sure why the examples you share would have any effect on this. Visibilty is only handled by the visible prop sent from the parent component. And the added code is in the componentWillUnmount life cycle method and so is only called when the component has been removed from the return of the parent's render function. What would it matter how the parent component determines the visibility prop it will pass?